Semiconductor device and manufacturing method thereof

ABSTRACT

A semiconductor device for high power application in which a novel semiconductor material having high mass productivity is provided. An oxide semiconductor film is formed, and then, first heat treatment is performed on the exposed oxide semiconductor film in order to reduce impurities such as moisture or hydrogen in the oxide semiconductor film. Next, in order to further reduce impurities such as moisture or hydrogen in the oxide semiconductor film, oxygen is added to the oxide semiconductor film by an ion implantation method, an ion doping method, or the like, and after that, second heat treatment is performed on the exposed oxide semiconductor film.

TECHNICAL FIELD

The present invention relates to a semiconductor device using an oxidesemiconductor and a manufacturing method thereof.

BACKGROUND ART

A transistor using a semiconductor film formed over an insulatingsurface is an essential semiconductor element for a semiconductordevice. Since the manufacturing of transistors has a limitation on theallowable temperature limit of a substrate, a transistor using, as anactive layer, amorphous silicon which can be formed at a relatively lowtemperature, polysilicon which can be obtained by crystallization usinglaser light or a catalytic element, or the like has been mainly used asa transistor for the semiconductor display device.

In recent years, a metal oxide having semiconductor characteristicswhich is referred to as an oxide semiconductor has attracted attentionas a novel semiconductor material which has both high mobility, which isa characteristic of polysilicon, and uniform element characteristics,which is a characteristic of amorphous silicon. The metal oxide has beenused for various applications; for example, indium oxide that is awell-known metal oxide has been used as a material of a transparentelectrode included in a liquid crystal display device or the like.Examples of such metal oxides having semiconductor characteristicsinclude tungsten oxide, tin oxide, indium oxide, and zinc oxide.Transistors in each of which a channel formation region is formed usingsuch a metal oxide having semiconductor characteristics have been known(Patent Documents 1 and 2).

REFERENCE Patent Document 1: Japanese Published Patent Application No.2007-123861 Patent Document 2: Japanese Published Patent Application No.2007-096055 DISCLOSURE OF INVENTION

As for a transistor included in a semiconductor device, it is preferableto reduce the variation of the threshold voltage caused by timedegradation and to reduce the off-state current. With the transistor thevariation of the threshold voltage caused by time degradation of whichis small, reliability of a semiconductor device can be increased. With atransistor the off-state current of which is low, power consumption of asemiconductor device can be suppressed.

It is an object of the present invention to provide a method formanufacturing a highly reliable semiconductor device. It is anotherobject of the present invention to provide a method for manufacturing asemiconductor device with low power consumption. It is another object ofthe present invention to provide a highly reliable semiconductor device.It is another object of the present invention to provide a semiconductordevice with low power consumption.

In a semiconductor device with high withstand voltage for controllinglarge current, a so-called power device, silicon has been used mainly asa semiconductor material. However, it is said that the physicalcharacteristics of a semiconductor element using silicon reach thetheoretical value limit, and a novel semiconductor material with whichthe characteristics can be improved has been demanded in order torealize a power device that has high withstand voltage and can controllarge current. As the semiconductor material that may improve thecharacteristics such as high withstand voltage, high conversionefficiency, or high-speed switching, for example, a compoundsemiconductor such as silicon carbide or gallium nitride can be given.The bandgap of silicon carbide and the bandgap of gallium nitride are3.26 eV and 3.39 eV, respectively, which are about three times as largeas that of silicon; it is known that such a compound semiconductor isadvantageous for improving the withstand voltage, reducing the powerloss of a semiconductor device, and the like.

On the other hand, the compound semiconductor such as silicon carbide orgallium nitride has a problem of high process temperature. The processtemperature of silicon carbide is about 1500° C. and the processtemperature of gallium nitride is about 1100° C., which does not allowfilm deposition on a glass substrate whose allowable temperature limitis low. Therefore, an inexpensive glass substrate cannot be used, andfurther, the compound semiconductor cannot be applied when the size of asubstrate is increased, so that the mass productivity of semiconductordevices using the compound semiconductor such as silicon carbide orgallium nitride is low, which disturbs practical application.

In view of the foregoing problems, an object of one embodiment of thepresent invention is to provide a semiconductor device for high powerapplication in which a novel semiconductor material having high massproductivity is used.

The present inventors paid their attention to the fact that impuritiessuch as hydrogen or water existing in an oxide semiconductor film causedegradation over time, such as shifts in threshold voltage, totransistors. It has been found that the oxide semiconductor film formedby sputtering or the like includes a large amount of hydrogen or wateras impurities. According to one embodiment of the present invention, inorder to decrease impurities such as moisture or hydrogen in the oxidesemiconductor film, after the oxide semiconductor film is formed, theexposed oxide semiconductor film is subjected to first heat treatment ina reduced-pressure atmosphere, an inert gas atmosphere of nitrogen, arare gas, or the like, an oxygen gas atmosphere, or an ultra dry airatmosphere (in air whose moisture content is less than or equal to 20ppm (dew point conversion, —55° C.), preferably less than or equal to 1ppm, far preferably less than or equal to 10 ppb in the case wheremeasurement is performed using a dew-point meter of a cavity ring-downlaser spectroscopy (CRDS) system). Next, in order to further decreaseimpurities such as moisture or hydrogen in the oxide semiconductor film,oxygen is added to the oxide semiconductor film by an ion implantationmethod, an ion doping method, or the like, and after that, the exposedoxide semiconductor film is subjected to second heat treatment in areduced-pressure atmosphere, an inert gas atmosphere of nitrogen, a raregas, or the like, an oxygen gas atmosphere, or an ultra dry airatmosphere (in air whose moisture content is less than or equal to 20ppm (dew point conversion, —55° C.), preferably less than or equal to 1ppm, far preferably less than or equal to 10 ppb in the case wheremeasurement is performed using a dew-point meter of a cavity ring-downlaser spectroscopy (CRDS) system).

The first heat treatment decreases the impurities such as moisture orhydrogen in the oxide semiconductor film, but does not eliminatecompletely, so that there is room for improvement. This incompletenessof impurity elimination is considered to be caused by hydrogen or ahydroxyl group bonded to a metal that is a component of the oxidesemiconductor. In accordance with the present invention, the bondbetween the metal as a component of the oxide semiconductor and thehydrogen or the hydroxyl group is cut and the hydrogen or the hydroxylgroup is made to react with oxygen to produce water by adding oxygen inthe oxide semiconductor film by an ion implantation method, an iondoping method, or the like. Then the second heat treatment is performedafter the addition of oxygen, whereby impurities such as hydrogen or ahydroxyl group left can be easily eliminated as water.

When an oxide semiconductor which is an i-type (intrinsic) semiconductoror a substantially i-type semiconductor can be obtained by eliminationof impurities such as moisture or hydrogen, deterioration ofcharacteristics of the transistor due to the impurities, such as shiftsin threshold voltage, can be prevented from being promoted and off-statecurrent can be reduced. Specifically, impurities such as hydrogen orwater contained in an oxide semiconductor are removed so that the valueof the concentration of hydrogen in the oxide semiconductor measured bysecondary ion mass spectroscopy (SIMS) is less than or equal to5×10¹⁹/cm³, preferably less than or equal to 5×10¹⁸/cm³, far preferablyless than or equal to 5×10¹⁷/cm³, still far preferably less than1×10¹⁶/cm³. In addition, the carrier density of the oxide semiconductorfilm, which is measured by Hall effect measurement, is less than1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, far preferably less than orequal to 1×10¹¹/cm³ that is less than or equal to a measurement limit.In other words, the carrier density of the oxide semiconductor film isextremely close to zero. Furthermore, the band gap of the oxidesemiconductor is greater than or equal to 2 eV, preferably greater thanor equal to 2.5 eV, far preferably greater than or equal to 3 eV. Withthe use of the oxide semiconductor film which is highly purified bysufficiently reducing the hydrogen concentration, the off-state currentof the transistor can be reduced.

The above two heat treatments are preferably performed at a temperaturehigher than or equal to 500° C. and lower than or equal to 850° C. (orlower than or equal to the strain point of a glass substrate), farpreferably at a temperature higher than or equal to 550° C. and lowerthan or equal to 750° C. Note that these heat treatments are performedat a temperature not exceeding the allowable temperature limit of thesubstrate to be used. An effect of elimination of water or hydrogen byheat treatment has been confirmed by thermal desorption spectroscopy(TDS).

Heat treatment in a furnace or a rapid thermal annealing method (RTAmethod) is used for the heat treatment. As the RTA method, a methodusing a lamp light source or a method in which heat treatment isperformed for a short time while a substrate is moved in a heated gascan be employed. With the use of the RTA method, it is also possible tomake the time involved in heat treatment shorter than 0.1 hours.

For example, even in the case where the transistor using the highlypurified oxide semiconductor film in the above-described manner is anelement whose channel width W is 1×10⁴ μm and whose channel length L is3 μm, electrical characteristics of an off-state current of 10⁻¹³ A orless and a subthreshold swing (S factor) of about 0.1 V/dec (thethickness of a gate insulating film is 100 nm) can be obtained.Therefore, the off-state current in the state where a source-drainvoltage is 0 or less, that is, the leakage current is much smaller thanthat of a transistor using silicon having crystallinity.

Further, such a transistor using a highly-purified oxide semiconductor(purified OS) exhibits almost no temperature dependence of off-statecurrent. This is because the conductivity type is made to be as close toan intrinsic type as possible by removing impurities which becomeelectron donors (donors) in the oxide semiconductor to highly purify theoxide semiconductor, so that the Fermi level positions in a center ofthe forbidden band. This also results from the fact that the oxidesemiconductor has an energy gap of 3 eV or more and includes very fewthermally excited carriers. In addition, the source electrode and thedrain electrode are in a degenerated state, which is also a factor forshowing no temperature dependence. Transistors are mainly operated withcarriers which are injected from the degenerated source electrode to theoxide semiconductor, and the above-described characteristics (nodependence of the off-state current on the temperature) can be explainedby no dependence of carrier density on the temperature.

Further, in the first heat treatment, by performing dehydration ordehydrogenation treatment at high temperature for short time on theoxide semiconductor film by an RTA (Rapid Thermal Anneal) method or thelike, a superficial portion of the oxide semiconductor film is made tohave a crystal region including a so-called nanocrystal with a grainsize greater than or equal to 1 nm and less than or equal to 20 nm, andthe other portion thereof is made to be amorphous or a mixture of anamorphous state and microcrystals where the microcrystals are providedin the amorphous state. Note that the above-described size of thenanocrystal is just an example, and the present invention is notconstrued as being limited to the above range.

The crystal region formed in the superficial portion of the oxidesemiconductor film is damaged by addition of oxygen by an ionimplantation method, an ion doping method, or the like. However, in theoxide semiconductor film, an oxygen defect is generated in addition toremoval of water or hydrogen by the first heat treatment, and oxygen canbe sufficiently supplied to the oxygen-deficient oxide semiconductorfilm by the addition of oxygen by an ion implantation method, an iondoping method, or the like. Further, since hydrogen or water removed bythe first heat treatment is not a component element of the oxidesemiconductor but a so-called impurity and oxygen added later is onecomponent element of the oxide semiconductor, a structure whichsatisfies the stoichiometric composition ratio can be obtained.Therefore, by performing the second heat treatment after the first heattreatment and the addition of oxygen, the damaged crystal region can berepaired, and crystal growth is promoted from the superficial portion ofthe oxide semiconductor film to a larger depth of the oxidesemiconductor film, so that the crystal region can be expanded. Further,crystal growth is further promoted by this second heat treatment ascompared to the first heat treatment, so that in the crystal region,crystal grains are adjacent to each other and metal elements each ofwhich is a component element of the oxide semiconductor are continuous,that is, continuously in contact with each other between crystal grainswhich are adjacent to each other. Accordingly, in a transistor in whicha channel formation region is formed using the above-described crystalregion, the potential barrier at a crystal grain boundary is low, sothat excellent characteristics such as high mobility and high withstandvoltage can be achieved.

As the oxide semiconductor, the following can be used: an oxide of fourmetal elements such as an In—Sn—Ga—Zn—O-based oxide semiconductor; anoxide of three metal elements such as an In—Ga—Zn—O-based oxidesemiconductor, an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga—Zn—O-based oxide semiconductor, or aSn—Al—Zn—O-based oxide semiconductor; an oxide of two metal elementssuch as an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, anIn—Mg—O-based oxide semiconductor, or an In—Ga—O-based oxidesemiconductor; an In—O-based oxide semiconductor, a Sn—O-based oxidesemiconductor, a Zn—O-based oxide semiconductor, or the like. In thisspecification, for example, an In—Sn—Ga—Zn—O-based oxide semiconductormeans a metal oxide including indium (In), tin (Sn), gallium (Ga), andzinc (Zn), whose stoichiometric composition ratio is not particularlylimited. The above-described oxide semiconductor may include silicon.

Alternatively, oxide semiconductors can be represented by the chemicalformula, InMO₃(ZnO)_(m) (m>0). Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co.

The analysis of the concentrations of hydrogen in the oxidesemiconductor film and the conductive film is described here. Theconcentrations of hydrogen in the oxide semiconductor film and theconductive film are measured by secondary ion mass spectrometry (SIMS).It is known that it is difficult to obtain data in the proximity of asurface of a sample or in the proximity of an interface between stackedfilms formed using different materials by the SIMS analysis inprinciple.

Thus, in the case where the distribution of the hydrogen concentrationof the film in a thickness direction is analyzed by SIMS, an averagevalue in a region of the film, in which the value is not greatly changedand almost the same value can be obtained is employed as the hydrogenconcentration. Further, in the case where the thickness of the film issmall, such a region where almost the same value can be obtained cannotbe found in some cases due to the influence of the hydrogenconcentration of a film which is adjacent to the film. In that case, themaximum value or the minimum value of the hydrogen concentration in theregion of the film is employed as the hydrogen concentration of thefilm. Further, in the case where a mountain-shaped peak having themaximum value or a valley-shaped peak having the minimum value do notexist in the region of the film, the value at the inflection point isemployed as the hydrogen concentration.

The transistor may be a bottom-gate transistor, a top-gate transistor,or a bottom-contact transistor. A bottom-gate transistor has a gateelectrode over an insulating surface; a gate insulating film over thegate electrode; an oxide semiconductor film which overlaps with the gateelectrode over the gate insulating film; a source electrode and a drainelectrode which are over the oxide semiconductor film; and an insulatingfilm over the source electrode, the drain electrode, and the oxidesemiconductor film. A top-gate transistor has an oxide semiconductorfilm over an insulating surface; a gate insulating film over the oxidesemiconductor film; a gate electrode which overlaps with the oxidesemiconductor film over the gate insulating film and functions as aconductive film; a drain electrode; a source electrode; and aninsulating film over the source electrode, the drain electrode, and theoxide semiconductor film. A bottom-contact transistor has a gateelectrode over an insulating surface; a gate insulating film over thegate electrode; a source electrode and a drain electrode which are overthe gate insulating film; an oxide semiconductor film which is over thesource electrode and the drain electrode and which overlaps with thegate electrode over the gate insulating film; and an insulating filmover the source electrode, the drain electrode, and the oxidesemiconductor film.

Hydrogen or water which is around the oxide semiconductor film is easilyabsorbed by the oxide semiconductor film not only in film deposition bysputtering or the like but also after the film deposition. Water orhydrogen easily forms a donor level and thus serves as an impurity inthe oxide semiconductor itself. Therefore, according to one embodimentof the present invention, after the source electrode and the drainelectrode are formed, an insulating film using an insulating materialhaving a high barrier property may be formed so as to cover the sourceelectrode, the drain electrode, and the oxide semiconductor film. It ispreferable to use an insulating material having a high barrier propertyfor the insulating film. For example, as the insulating film having ahigh barrier property, a silicon nitride film, a silicon nitride oxidefilm, an aluminum nitride film, an aluminum nitride oxide film, or thelike can be used. When a plurality of insulating films stacked is used,an insulating film having lower proportion of nitrogen than theinsulating film having a high barrier property, such as a silicon oxidefilm or a silicon oxynitride film, is formed on the side close to theoxide semiconductor film. Then, an insulating film having a barrierproperty is formed so as to overlap with the source electrode, the drainelectrode, and the oxide semiconductor film with the insulating filmhaving lower proportion of nitrogen provided therebetween. With theinsulating film having a barrier property, impurities such as moistureor hydrogen can be prevented from entering the oxide semiconductor film,the gate insulating film, or the interface between the oxidesemiconductor film and another insulating film and the vicinity thereof.

Between the gate electrode and the oxide semiconductor film, a gateinsulating film may be formed to have a structure in which an insulatingfilm formed using a material having a high barrier property and aninsulating film having lower proportion of nitrogen, such as a siliconoxide film or a silicon oxynitride film are stacked. The insulating filmsuch as a silicon oxide film or a silicon oxynitride film is formedbetween the insulating film having a barrier property and the oxidesemiconductor film. With the insulating film having a barrier property,impurities in an atmosphere, such as moisture or hydrogen, or impuritiesincluded in a substrate, such as an alkali metal or a heavy metal, canbe prevented from entering the oxide semiconductor film, the gateinsulating film, or the interface between the oxide semiconductor filmand another insulating film and the vicinity thereof.

In accordance with the present invention, a method for manufacturing asemiconductor device with high reliability can be provided. A method formanufacturing a semiconductor device with low power consumption can beprovided. Further, a semiconductor device with high reliability can beprovided. A semiconductor device with low power consumption can beprovided.

Further, a semiconductor element having a high withstand voltage can bemanufactured at a low film deposition temperature, so that asemiconductor device for high power application with high massproductivity can be provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1E are views illustrating a method for manufacturing asemiconductor device;

FIGS. 2A to 2C are views illustrating a method for manufacturing asemiconductor device;

FIGS. 3A to 3C are views illustrating a method for manufacturing asemiconductor device;

FIG. 4 is a cross-sectional view of a semiconductor device;

FIGS. 5A to 5E are views illustrating a method for manufacturing asemiconductor device;

FIGS. 6A to 6C are views illustrating a method for manufacturing asemiconductor device;

FIGS. 7A and 7B are top views of a semiconductor device;

FIGS. 8A to 8C are views illustrating a method for manufacturing asemiconductor device;

FIG. 9 is a top view of a semiconductor device;

FIGS. 10A to 10C are views illustrating a method for manufacturing asemiconductor device;

FIGS. 11A and 11B are cross-sectional views of a transistor;

FIGS. 12A and 12B are cross-sectional views of a transistor;

FIGS. 13A and 13B are a top view and a cross-sectional view ofelectronic paper, respectively;

FIGS. 14A and 14B are block diagrams each of a semiconductor device;

FIGS. 15A and 15B illustrate a structure of a signal line drivercircuit;

FIGS. 16A and 16B are circuit diagrams each illustrating a structure ofa shift register;

FIGS. 17A and 17B are a diagram illustrating one embodiment of a shiftregister and a timing chart illustrating operation thereof,respectively;

FIG. 18 is a cross-sectional view of a liquid crystal display device;

FIG. 19 is a view illustrating a structure of a liquid crystal displaydevice module;

FIGS. 20A to 20C are cross-sectional views each of a light-emittingdevice;

FIGS. 21A to 21F are views each illustrating an electronic device usinga semiconductor device;

FIG. 22 is a longitudinal cross-sectional view of an inverted staggeredtransistor using an oxide semiconductor;

FIG. 23 is an energy band diagram (an schematic diagram) along thesection A-A′ of FIG. 22 ;

FIG. 24A illustrates the state where a positive potential (VG>0) isapplied to a gate electrode (GE) and FIG. 24B illustrates the statewhere a negative potential (VG<0) is applied to the gate electrode (GE);and

FIG. 25 illustrates a relation between the vacuum level and the workfunction of a metal (ϕ_(M)), and a relation between the vacuum level andthe electron affinity of an oxide semiconductor (χ).

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments and Example of the present invention will be described belowwith reference to the accompanying drawings. Note that the presentinvention is not limited to the following description, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionis not to be construed as being limited to the content of theembodiments included herein.

The present invention can be applied to manufacture of any kind ofsemiconductor devices including microprocessors, integrated circuitssuch as image processing circuits, RF tags, and semiconductor displaydevices. A semiconductor device means any device which can function byutilizing semiconductor characteristics, and a semiconductor displaydevice, a semiconductor circuit, and an electronic device are allincluded in the category of the semiconductor device. The semiconductordisplay devices include the following in its category: liquid crystaldisplay devices, light-emitting devices in which a light-emittingelement typified by an organic light-emitting element (OLED) is providedfor each pixel, electronic papers, digital micromirror devices (DMDs),plasma display panels (PDPs), field emission displays (FEDs), and othersemiconductor display devices in which a circuit element using asemiconductor film is included in a driver circuit.

Embodiment 1

A bottom-gate transistor having a channel-etched structure is taken asan example, and a structure of the transistor included in asemiconductor device according to one embodiment of the presentinvention and a manufacturing method thereof will be described.

As shown in FIG. 1A, a gate electrode 101 is formed over a substrate100.

Although there is no particular limitation on a substrate that can beused as the substrate 100 having an insulating surface, it is necessarythat the substrate has heat resistance high enough to withstand at leastheat treatment to be performed later. For example, a glass substratemanufactured by a fusion method or a float method can be used. When thetemperature of the heat treatment performed later is high, it ispreferable to use a substrate having a strain point of 730° C. or higheras the glass substrate. For the glass substrate, a glass material suchas aluminosilicate glass, aluminoborosilicate glass, or bariumborosilicate glass is used, for example. In general, by containing alarger amount of barium oxide (BaO) than boron oxide, a glass substratewhich is heat-resistant and more practical can be obtained. Therefore,it is preferable to use a glass substrate containing BaO and B₂O₃ sothat the amount of BaO is larger than that of B₂O₃.

Instead of the above-described glass substrate, a substrate formed of aninsulator such as a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used. Alternatively, crystallized glass or the like maybe used. Further alternatively, a metal substrate such as a substrate ofa stainless steel alloy, provided with an insulating film on itssurface, may be used.

A substrate formed from a flexible synthetic resin, such as plastic,generally tends to have a low allowable temperature limit, but can beused as the substrate 100 as long as the substrate can withstand aprocessing temperature in the later manufacturing process. Examples ofthe plastic substrate include polyester typified by polyethyleneterephthalate (PET), polyethersulfone (PES), polyethylene naphthalate(PEN), polycarbonate (PC), polyetheretherketone (PEEK), polysulfone(PSF), polyetherimide (PEI), polyarylate (PAR), polybutyleneterephthalate (PBT), polyimide, acrylonitrile-butadiene-styrene resin,polyvinyl chloride, polypropylene, polyvinyl acetate, acrylic resin, andthe like.

An insulating film to serve as a base film may be formed between thesubstrate 100 and the gate electrode 101. As the base film, for example,a single layer of a silicon oxide film, a silicon oxynitride film, asilicon nitride film, a silicon nitride oxide film, an aluminum nitridefilm, or an aluminum nitride oxide film or a stacked layer of aplurality of these films can be used. In particular, an insulating filmhaving a high barrier property, for example, a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, or an aluminumnitride oxide film may be used for the base film, so that impurities inan atmosphere, such as moisture or hydrogen, or impurities included inthe substrate 100, such as an alkali metal or a heavy metal, can beprevented from entering the oxide semiconductor film, the gateinsulating film, or the interface between the oxide semiconductor filmand another insulating film and the vicinity thereof.

In this specification, oxynitride refers to a substance which includesmore oxygen than nitrogen, and nitride oxide refers to a substance whichincludes more nitrogen than oxygen.

The gate electrode 101 can be formed to have a single-layer structure ora stacked-layer structure using one or more conductive films using ametal material such as molybdenum, titanium, chromium, tantalum,tungsten, neodymium, or scandium or an alloy material which contains anyof these metal materials as its main component, or a nitride whichcontains any of these metals. Aluminum or copper can be used as one ofthe metal material if aluminum or copper can withstand a temperature ofheat treatment performed in a later process. Aluminum or copper ispreferably combined with a refractory metal material so as to prevent aheat resistance problem and a corrosive problem. As the refractory metalmaterial, molybdenum, titanium, chromium, tantalum, tungsten, neodymium,scandium, or the like can be used.

For example, as a two-layer structure of the gate electrode 101, thefollowing structure is preferable: a two-layer structure in which amolybdenum film is stacked over an aluminum film, a two-layer structurein which a molybdenum film is stacked over a copper film, a two-layerstructure in which a titanium nitride film or a tantalum nitride film isstacked over a copper film, or a two-layer structure in which a titaniumnitride film and a molybdenum film are stacked. As a three-layerstructure of the gate electrode 101, the following structure ispreferable: a stacked structure in which an aluminum film, an alloy filmof aluminum and silicon, an alloy film of aluminum and titanium, or analloy film of aluminum and neodymium is sandwiched by any two filmsselected from a tungsten film, a tungsten nitride film, a titaniumnitride film, and a titanium film.

Further, a light-transmitting oxide conductive film of indium oxide, analloy of indium oxide and tin oxide, an alloy of indium oxide and zincoxide, zinc oxide, aluminum zinc oxide, aluminum zinc oxynitride,gallium zinc oxide, or the like may be used for the gate electrode 101,so that the aperture ratio of a pixel portion can be increased.

The gate electrode 101 is formed to a thickness of 10 nm to 400 nm,preferably 100 nm to 200 nm. In this embodiment, a conductive film forthe gate electrode is formed to a thickness of 150 nm by a sputteringmethod using a tungsten target, and then, the conductive film isprocessed (patterned) into an appropriate shape by etching; in such amanner, the gate electrode 101 is formed. It is preferable that an endportion of the gate electrode be tapered because coverage with a gateinsulating film formed thereover is improved. Note that a resist maskmay be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing costs can bereduced.

Next, a gate insulating film 102 is formed over the gate electrode 101.The gate insulating film 102 can be formed to have a single-layerstructure or a stacked-layer structure of one or more selected from asilicon oxide film, a silicon nitride film, a silicon oxynitride film, asilicon nitride oxide film, an aluminum oxide film, an aluminum nitridefilm, an aluminum oxynitride film, an aluminum nitride oxide film, ahafnium oxide film, and a tantalum oxide film by a plasma CVD method, asputtering method, or the like. It is preferable that the gateinsulating film 102 include impurities such as moisture or hydrogen aslittle as possible. In the case where a silicon oxide film is formed bya sputtering method, a silicon target or a quartz target is used as atarget and oxygen or a mixed gas of oxygen and argon is used as asputtering gas.

An oxide semiconductor that is made to be i-type or substantially i-type(an oxide semiconductor that is highly purified) by removal ofimpurities is extremely sensitive to an interface state and an interfaceelectric charge; thus, an interface between the oxide semiconductor andthe gate insulating film 102 is important. Thus, a gate insulating film(GI) which is to be in contact with the highly purified oxidesemiconductor needs to have high quality.

For example, high-density plasma CVD using microwaves (2.45 GHz) ispreferable because a dense high-quality insulating film having highwithstand voltage can be formed. This is because an interface state canbe reduced and interface characteristics can be favorable when thehighly purified oxide semiconductor and the high quality gate insulatingfilm are in contact with each other.

Needless to say, other film formation methods, such as a sputteringmethod or a plasma CVD method, can be applied as long as a high-qualityinsulating film can be formed as the gate insulating film. The filmquality of the gate insulating film and/or properties of an interfacewith an oxide semiconductor thereof may be modified by heat treatmentperformed after film deposition. In any case, any insulating film can beused as long as film quality as a gate insulating film is high,interface state density with an oxide semiconductor is decreased, and afavorable interface can be formed.

The gate insulating film 102 may have a structure in which an insulatingfilm formed using a material having a high barrier property and aninsulating film having lower proportion of nitrogen such as a siliconoxide film or a silicon oxynitride film are stacked. In that case, theinsulating film such as a silicon oxide film or a silicon oxynitridefilm is formed between the insulating film having a barrier property andthe oxide semiconductor film. As the insulating film having a highbarrier property, a silicon nitride film, a silicon nitride oxide film,an aluminum nitride film, an aluminum nitride oxide film, or the likecan be given, for example. With the insulating film having a barrierproperty, impurities in an atmosphere, such as moisture or hydrogen, orimpurities included in the substrate, such as an alkali metal or a heavymetal, can be prevented from entering the oxide semiconductor film, thegate insulating film 102, or the interface between the oxidesemiconductor film and another insulating film and the vicinity thereof.In addition, the insulating film having lower proportion of nitrogensuch as a silicon oxide film or a silicon oxynitride film may be formedso as to be in contact with the oxide semiconductor film, so that theinsulating film formed using a material having a high barrier propertycan be prevented from being in contact with the oxide semiconductor filmdirectly.

For example, a stacked-layer film with a thickness of 100 nm may beformed as the gate insulating film 102 as follows: a silicon nitridefilm (SiN_(y) (y>0)) with a thickness greater than or equal to 50 nm andless than or equal to 200 nm is formed by a sputtering method as a firstgate insulating film, and a silicon oxide film (SiO_(x) (x>0)) with athickness greater than or equal to 5 nm and less than or equal to 300 nmis stacked over the first gate insulating film as a second gateinsulating film. The thickness of the gate insulating film 102 may beset as appropriate depending on characteristics needed for a transistorand may be about 350 nm to 400 nm.

In this embodiment, the gate insulating film 102 is formed to have astructure in which a silicon oxide film having a thickness of 100 nmformed by a sputtering method is stacked over a silicon nitride filmhaving a thickness of 50 nm formed by a sputtering method.

In order that hydrogen, a hydroxyl group, and moisture may be containedin the gate insulating layer 102 as little as possible, it is preferablethat the substrate 100 over which the gate electrode layer 101 is formedbe preheated in a preheating chamber of a sputtering apparatus aspretreatment for film formation so that impurities such as hydrogen andmoisture adsorbed to the substrate 100 are removed and exhausted. Thetemperature of the preheating is higher than or equal to 100° C. andlower than or equal to 400° C., preferably higher than or equal to 150°C. and lower than or equal to 300° C. As an exhaustion unit provided inthe preheating chamber, a cryopump is preferable. This preheatingtreatment can be omitted.

Next, over the gate insulating film 102, an oxide semiconductor film 103having a thickness greater than or equal to 2 nm and less than or equalto 200 nm, preferably greater than or equal to 3 nm and less than orequal to 50 nm, far preferably greater than or equal to 3 nm and lessthan or equal to 20 nm is formed. The oxide semiconductor film 103 isformed by a sputtering method using an oxide semiconductor as a target.The oxide semiconductor film 103 can be formed by a sputtering methodunder a rare gas (for example, argon) atmosphere, an oxygen atmosphere,or an atmosphere including a rare gas (for example, argon) and oxygen.

Before the oxide semiconductor film 103 is formed by a sputteringmethod, dust attached to a surface of the gate insulating film 102 ispreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which, without application of a voltage to a target side, anRF power source is used for application of a voltage to a substrate sidein an argon atmosphere to modify a surface. Instead of an argonatmosphere, a nitrogen atmosphere, a helium atmosphere, or the like maybe used. Alternatively, an argon atmosphere to which oxygen, nitrousoxide, or the like is added may be used. Further alternatively, an argonatmosphere to which chlorine, carbon tetrafluoride, or the like is addedmay be used.

The above-described oxide semiconductor can be used as the oxidesemiconductor film 103.

In this embodiment, as the oxide semiconductor film 103, anIn—Ga—Zn—O-based non-single-crystal film with a thickness of 30 nm,which is obtained by a sputtering method using an oxide semiconductortarget including indium (In), gallium (Ga), and zinc (Zn), is used. Asthe target, for example, an oxide semiconductor target having acomposition ratio with an atom ratio of metals, In:Ga:Zn=1:1:0.5,In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used. Further, the oxidesemiconductor film 103 can be formed by a sputtering method under a raregas (typically argon) atmosphere, an oxygen atmosphere, or an atmospherecontaining a rare gas (typically argon) and oxygen. In the case of usinga sputtering method, a target containing SiO₂ at 2 wt % to 10 wt % bothinclusive may be used for depositing the film. The filling rate of theoxide semiconductor target including In, Ga, and Zn is greater than orequal to 90% and less than or equal to 100%, preferably greater than orequal to 95% and less than or equal to 99.9%. With the use of the oxidesemiconductor target with a high filling rate, a dense oxidesemiconductor film is formed.

The oxide semiconductor film 103 is formed over the substrate 100 insuch a manner that the substrate is held in the treatment chambermaintained at reduced pressure, a sputtering gas from which hydrogen andmoisture have been removed is introduced into the treatment chamberwhile moisture remaining therein is removed, and metal oxide is used asa target. At that time, the substrate may be heated at higher than orequal to 100° C. and lower than or equal to 600° C., preferably higherthan or equal to 200° C. and lower than or equal to 400° C. Filmdeposition may be performed while the substrate is heated, whereby theconcentration of an impurity contained in the oxide semiconductor filmdeposited can be reduced. In addition, damage by sputtering can bereduced. In order to remove remaining moisture in the treatment chamber,an entrapment vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. Anevacuation unit may be a turbo pump provided with a cold trap. In thedeposition chamber which is evacuated with the cryopump, for example, ahydrogen atom, a compound containing a hydrogen atom, such as water(H₂O), (far preferably, also a compound containing a carbon atom), andthe like are removed, whereby the concentration of an impurity in theoxide semiconductor film deposited in the deposition chamber can bereduced.

As one example of the deposition condition, the following can beemployed: the distance between the substrate and the target is 100 mm,the pressure is 0.6 Pa, the direct-current (DC) power is 0.5 kW, and theatmosphere is an oxygen atmosphere (the proportion of the oxygen flowrate is 100%). Note that a pulsed direct-current (DC) power source ispreferable because powder substances (also referred to as particles)generated in film deposition can be reduced and the film thickness canbe uniform. The oxide semiconductor film preferably has a thicknessgreater than or equal to 5 nm and less than or equal to 30 nm. Sinceappropriate thickness depends on an oxide semiconductor material used,the thickness can be determined as appropriate depending on thematerial.

Further, in order that hydrogen, a hydroxyl group, and moisture may becontained in the oxide semiconductor film 103 as little as possible, itis preferable that the substrate 100 on which the process up to andincluding the step of forming the gate insulating film 102 is alreadyperformed be preheated in a preheating chamber of a sputtering apparatusas pretreatment for film formation so that impurities such as hydrogenand moisture adsorbed to the substrate 100 are removed and exhausted.The temperature of the preheating is higher than or equal to 100° C. andlower than or equal to 400° C., preferably higher than or equal to 150°C. and lower than or equal to 300° C. As an exhaustion unit provided inthe preheating chamber, a cryopump is preferable. This preheatingtreatment can be omitted. Further, this preheating may be similarlyperformed on the substrate 100 on which the process up to and includingthe step of forming a source electrode 111 and a drain electrode 112 isalready performed, before the formation of an insulating film 113.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power supply, aDC sputtering method, and a pulsed DC sputtering method in which a biasis applied in a pulsed manner. An RF sputtering method is mainly used inthe case where an insulating film is formed, and a DC sputtering methodis mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

Alternatively, a sputtering apparatus provided with a magnet systeminside the chamber and used for a magnetron sputtering method, or asputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge can be used.

Further, as a deposition method using a sputtering method, a reactivesputtering method in which a target substance and a sputtering gascomponent are chemically reacted with each other during deposition toform a thin compound film thereof, or a bias sputtering method in whicha voltage is also applied to a substrate during deposition can be used.

The gate insulating film 102 and the oxide semiconductor film 103 may beformed successively without exposure to air. Successive film formationwithout exposure to air makes it possible to obtain each interfacebetween stacked layers, which is not contaminated by atmosphericcomponents or impurity elements floating in air, such as water,hydrocarbon, or the like. Accordingly, variation in characteristics ofthe transistor can be reduced.

Next, as illustrated in FIG. 1B, the oxide semiconductor film 103 isprocessed (patterned) into an appropriate shape by etching or the like,whereby an island-shaped oxide semiconductor film 104 is formed over thegate insulating film 102 so as to overlap with the gate electrode 101.

A resist mask for forming the island-shaped oxide semiconductor film 104may be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing costs can bereduced.

In the case where a contact hole is formed in the gate insulating film102, a step of forming the contact hole can be performed at the time offormation of the island-shaped oxide semiconductor film 104.

The etching for forming the island-shaped oxide semiconductor film 104may be dry etching, wet etching, or both dry etching and wet etching. Asthe etching gas for the dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) ispreferably used. Alternatively, a gas containing fluorine(fluorine-based gas such as carbon tetrafluoride (CF₄), sulfurhexafluoride (SF₆), nitrogen trifluoride (NF₃), or trifluoromethane(CHF₃)); hydrogen bromide (HBr); oxygen (O₂); any of these gases towhich a rare gas such as helium (He) or argon (Ar) is added; or the likecan be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the film into an appropriate shape, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

As an etchant used for the wet etching, a mixed solution of phosphoricacid, acetic acid, and nitric acid, organic acid such as citric acid oroxalic acid, or the like can be used. Alternatively, ITO-07N(manufactured by Kanto Chemical Co., Inc.) may be used. The etchantafter the wet etching is removed together with the etched materials bycleaning. The waste liquid including the etchant and the material etchedoff may be purified and the material may be reused. A material such asindium included in the oxide semiconductor film may be collected fromthe waste liquid after the etching and reused, whereby the resources canbe efficiently used and the costs can be reduced.

It is preferable that reverse sputtering be performed before theformation of a conductive film in a subsequent step so that a resistresidue or the like that attaches onto surfaces of the island-shapedoxide semiconductor film 104 and the gate insulating film 102 isremoved.

Next, first heat treatment is performed on the oxide semiconductor film104 in a reduced-pressure atmosphere, an inert gas atmosphere such as anitrogen atmosphere or a rare gas atmosphere, an oxygen gas atmosphere,or an ultra dry air atmosphere (in air whose moisture content is lessthan or equal to 20 ppm (dew point conversion, —55° C.), preferably lessthan or equal to 1 ppm, far preferably less than or equal to 10 ppb inthe case where measurement is performed using a dew-point meter of acavity ring-down laser spectroscopy (CRDS) system). By the first heattreatment on the oxide semiconductor film 104, an oxide semiconductorfilm 105 in which moisture or hydrogen is eliminated is formed as shownin FIG. 1C. Specifically, heat treatment may be performed at atemperature higher than or equal to 500° C. and lower than or equal to850° C. (or a temperature lower than or equal to a strain point of aglass substrate), preferably a temperature higher than or equal to 550°C. and lower than or equal to 750° C. For example, heat treatment may beperformed at 600° C. for a period longer than or equal to 3 minutes andshorter than or equal to 6 minutes. Since dehydration or dehydrogenationcan be performed in a short time with the RTA method, the first heattreatment can be performed even at a temperature over the strain pointof a glass substrate. In this embodiment, heat treatment is performed onthe oxide semiconductor film 104 at a substrate temperature of 600° C.for 6 minutes in a nitrogen atmosphere with the use of an electricalfurnace that is one of heat treatment apparatuses, and then, the oxidesemiconductor film is not exposed to the air and water and hydrogen areprevented from entering the oxide semiconductor film, so that the oxidesemiconductor film 105 is obtained.

The heat treatment apparatus is not limited to an electrical furnace,and may be provided with a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermal anneal)apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA(lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas,an inert gas which does not react with an object to be processed by heattreatment, such as nitrogen or a rare gas such as argon is used.

For example, as the first heat treatment, GRTA by which the substrate ismoved into an inert gas heated to a high temperature as high as 650° C.to 700° C., heated for several minutes, and moved out of the inert gasheated to the high temperature may be performed. GRTA enableshigh-temperature heat treatment for a short period of time.

It is preferable that in the heat treatment, moisture, hydrogen, or thelike be not contained in nitrogen or a rare gas such as helium, neon, orargon. The purity of nitrogen or the rare gas such as helium, neon, orargon which is introduced into the heat treatment apparatus ispreferably set to be 6N (99.9999%) or higher, far preferably 7N(99.99999%) or higher (that is, the impurity concentration is preferably1 ppm or lower, far preferably 0.1 ppm or lower).

Thus, as shown in FIG. 1C, by the first heat treatment, a superficialportion of the island-shaped oxide semiconductor film 105 is made tohave a crystal region 106. The crystal region 106 includes a so-callednanocrystal with a grain size greater than or equal to 1 nm and lessthan or equal to 20 nm, and the other portion of the island-shaped oxidesemiconductor film 105 other than the crystal region 106 is amorphous orincludes a mixture of an amorphous state and microcrystals where themicrocrystals are provided in the amorphous state. Note that theabove-described size of the nanocrystal is just an example, and thepresent invention is not construed as being limited to the above range.In the case where the oxide semiconductor film is an In—Ga—Zn—O-basedoxide semiconductor film formed by a sputtering method using a targetthe atom ratio of metals of which is In:Ga:Zn=1:1:1, crystallization ofthe superficial portion of the oxide semiconductor film is likely to befurther promoted as compared to the case where a target having adifferent atom ratio is used, so that the crystal region 106 is likelyto be formed to a larger depth.

Next, as shown in FIG. 1D, oxygen is added into the oxide semiconductorfilm having the crystal region 106 in the superficial portion by an ionimplantation method or an ion doping method. Oxygen is added into theoxide semiconductor film 105 by an ion implantation method, an iondoping method, or the like, so that an oxide semiconductor film 107 inwhich oxygen is added excessively is formed. By adding oxygen, a bondbetween a metal as a component of the oxide semiconductor and hydrogenor a bond between the metal and a hydroxyl group is cut and the hydrogenor the hydroxyl group is reacted with oxygen to produce water; thisleads to easy elimination of hydrogen or a hydroxyl group that is animpurity, in the form of water by second heat treatment performed later.

In an ion implantation method, a source gas is made into plasma, ionspecies included in this plasma are extracted and mass-separated, ionspecies having predetermined mass are accelerated, and an object isirradiated with the accelerated ion species in the form of an ion beam.In an ion doping method, a source gas is made into plasma, ion speciesare extracted from this plasma by an operation of a predeterminedelectric field, the extracted ion species are accelerated without massseparation, and an object is irradiated with the accelerated ion speciesin the form of an ion beam. When the addition of oxygen is performedusing an ion implantation method involving mass-separation, an impuritysuch as a metal element can be prevented from being added into the oxidesemiconductor film. On the other hand, an ion doping method enablesion-beam irradiation to a larger area than an ion implantation method,and therefore, when the addition of oxygen is performed using an iondoping method, the takt time can be shortened.

In the case where an oxygen gas is used and oxygen is added by an ionimplantation method, the acceleration voltage may be set in the range of5 kV to 100 kV both inclusive and the dosage may be set in the range of1×10¹³ ions/cm² to 1×10¹⁶ ions/cm² both inclusive.

Heat treatment may be performed on the substrate provided with the oxidesemiconductor film 105 at a temperature higher than or equal to 500° C.and lower than or equal to 850° C. (or a temperature lower than or equalto a strain point of a glass substrate), preferably a temperature higherthan or equal to 550° C. and lower than or equal to 750° C. while theaddition of oxygen into the oxide semiconductor film 105 is performed byan ion implantation method.

Crystals included in the crystal region 106 formed in the superficialportion of the oxide semiconductor film 105 are damaged by addition ofoxygen using an ion implantation method, an ion doping method, or thelike. Therefore, the crystallinity of a superficial portion of the oxidesemiconductor film 107 is lower than that of the crystal region 106 ofthe oxide semiconductor film 105 before the oxygen addition, and thesuperficial portion of the oxide semiconductor film 107 may be in asimilar state to the amorphous region of the oxide semiconductor film105 depending on the dosage of oxygen.

Next, second heat treatment is performed. The second heat treatment canbe performed in a similar condition to the first heat treatment.Specifically, heat treatment may be performed in a reduced-pressureatmosphere, an inert gas atmosphere such as a nitrogen atmosphere or arare gas atmosphere, an oxygen gas atmosphere, or an ultra dry airatmosphere (in air whose moisture content is less than or equal to 20ppm (dew point conversion, —55° C.), preferably less than or equal to 1ppm, far preferably less than or equal to 10 ppb in the case wheremeasurement is performed using a dew-point meter of a cavity ring-downlaser spectroscopy (CRDS) system). In the case where heat treatment isperformed by RTA (Rapid Thermal Anneal), for example, heat treatment maybe performed at 600° C. for a period longer than or equal to 3 minutesand shorter than or equal to 6 minutes. Since dehydration ordehydrogenation can be performed in a short time with the RTA method,the second heat treatment can be performed even at a temperature overthe strain point of a glass substrate. In this embodiment, heattreatment is performed at a substrate temperature of 600° C. for 6minutes in a nitrogen atmosphere with the use of an electrical furnacethat is one of heat treatment apparatuses, and then, the oxidesemiconductor film is not exposed to the air and water and hydrogen areprevented from entering the oxide semiconductor film, so that an oxidesemiconductor film 108 is obtained as shown in FIG. 1E. The heattreatment may be performed plural times after the island-shaped oxidesemiconductor film 108 is formed.

According to one embodiment of the present invention, the bond between ametal as a component of the oxide semiconductor and hydrogen or ahydroxyl group is cut and the hydrogen or the hydroxyl group is made toreact with oxygen to produce water by adding oxygen in the oxidesemiconductor film 105. Thus, an impurity such as hydrogen or a hydroxylgroup left in the film can be easily eliminated in the form of water bythe second heat treatment after the oxygen addition. The island-shapedoxide semiconductor film 108 formed through the second heat treatment ismore i-type (intrinsic) or closer to i-type than the oxide semiconductorfilm 105 after the first heat treatment because impurities such asmoisture or hydrogen left even after the first heat treatment areremoved. Impurities such as moisture or hydrogen are eliminated, and theisland-shaped oxide semiconductor film becomes an i-type (intrinsic)semiconductor or a substantially i-type semiconductor; therefore,deterioration of characteristics of the transistor due to theimpurities, such as shifts in threshold voltage, can be prevented frombeing promoted and off-state current can be reduced.

Further, when an oxide semiconductor containing an impurity is subjectedto a gate bias-temperature stress test (BT test) for 12 hours underconditions that the temperature is 85° C. and the voltage applied to thegate is 2×10⁶ V/cm, a bond between the impurity and a main component ofthe oxide semiconductor is cut by a high electric field (B: bias) and ahigh temperature (T: temperature), and a generated dangling bond inducesdrift of threshold voltage (V_(th)). However, in the above-describedmanner, by improving the interfacial characteristics between the gateinsulating film and the oxide semiconductor film and removingimpurities, particularly hydrogen, water, and the like, in the oxidesemiconductor film as much as possible, a transistor which remainsstable even with respect to the BT test can be obtained.

The heat treatment apparatus is not limited to an electrical furnace,and may be provided with a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermal anneal)apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA(lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas,an inert gas which does not react with an object to be processed by heattreatment, such as nitrogen or a rare gas such as argon is used.

For example, as the second heat treatment, GRTA by which the substrateis moved into an inert gas heated to a high temperature as high as 650°C. to 700° C., heated for several minutes, and moved out of the inertgas heated to the high temperature may be performed. GRTA enableshigh-temperature heat treatment for a short period of time.

It is preferable that in the heat treatment, moisture, hydrogen, or thelike be not contained in nitrogen or a rare gas such as helium, neon, orargon. The purity of nitrogen or the rare gas such as helium, neon, orargon which is introduced into the heat treatment apparatus ispreferably set to be 6N (99.9999%) or higher, far preferably 7N(99.99999%) or higher (that is, the impurity concentration is preferably1 ppm or lower, far preferably 0.1 ppm or lower).

However, in the oxide semiconductor film 105, an oxygen defect isgenerated in addition to removal of water or hydrogen by the first heattreatment, and oxygen can be sufficiently supplied to theoxygen-deficient oxide semiconductor film by the addition of oxygen byan ion implantation method, an ion doping method, or the like. Further,since hydrogen or water removed by the first heat treatment is not acomponent element of the oxide semiconductor but a so-called impurityand oxygen added later is one component element of the oxidesemiconductor, a structure which satisfies the stoichiometriccomposition ratio can be obtained. Therefore, by performing the secondheat treatment after the first heat treatment and the addition ofoxygen, the damaged crystal region 106 can be repaired, and crystalgrowth is promoted from the superficial portion of the oxidesemiconductor film 108 to a larger depth of the oxide semiconductorfilm, so that a crystal region 109 which is expanded to a deeper portionof the oxide semiconductor film 108 as shown in FIG. 1E can be formed.Further, crystal growth is further promoted by this second heattreatment as compared to the first heat treatment, so that in thecrystal region 109, crystal grains are adjacent to each other and metalelements each of which is a component element of the oxide semiconductorare continuous, that is, continuously in contact with each other betweencrystal grains which are adjacent to each other.

The crystal region 109 is described below in more detail. C-axes ofcrystals of the crystal region 109 in the superficial portion arealigned in a direction which is almost perpendicular to the top surfaceof the oxide semiconductor film 108 and adjacent to each other. Forexample, in the case of using an In—Ga—Zn—O-based oxide semiconductormaterial, in the crystal region 109, the c-axes of crystals of InGaZnO₄are oriented in a direction which is almost perpendicular to the topsurface of the oxide semiconductor film 108.

The crystals of InGaZnO₄ include any of In, Ga, and Zn, and can beconsidered to have a stacked-layer structure of layers parallel to ana-axis and a b-axis. That is, the crystals of InGaZnO₄ have a structurein which a first layer including In, a second layer including In, and athird layer including In are stacked in a c-axis direction.

Since electrical conductivity of the crystals of InGaZnO₄ are controlledmainly by In, electrical characteristics of the first layer including Into the third layer including In in a planar direction parallel to thea-axis and the b-axis are preferable. This is because one of a 5 sorbital of In is overlapped with a 5 s orbital of which are adjacent toIn in at least one of the first to third layers including In, so that acarrier path is formed.

When such crystals are oriented, an effect on electrical characteristicsof the oxide semiconductor film 108 also arises. Specifically, forexample, electrical characteristics in a direction parallel to the topsurface of the oxide semiconductor film 108 are improved. This isbecause the c-axes of the crystals of InGaZnO₄ are oriented in adirection almost perpendicular to the top surface of the oxidesemiconductor film 108, and current flows in a planar direction parallelto the a-axis and the b-axis in InGaZnO₄ crystals.

According to one embodiment of the present invention, in the crystalregion, crystal grains adjacent to each other and metal elements each ofwhich is a component element of the oxide semiconductor are continuous,that is, continuously in contact with each other between crystal grainswhich are adjacent to each other. Therefore, current is likely to easilyflow in directions parallel to the a-axis and b-axis, denoted by arrowsin FIG. 4 , so that electrical characteristics in a direction parallelto the top surface of the oxide semiconductor film 108 can be furtherimproved. The oxide semiconductor film 108 shown in FIG. 1E includes anamorphous region 110 which is mainly amorphous and the crystal region109 formed in the superficial portion of the oxide semiconductor film108.

The crystal structure of the crystal region 109 is not limited to theabove-described structure, and the crystal region 109 may include acrystal having another structure. For example, in the case of using anIn—Ga—Zn—O-based oxide semiconductor material, crystals of In₂Ga₂ZnO₇,InGaZn₅O₈, or the like may be included in addition to crystals ofInGaZnO₄ crystals. Needless to say, the case where the crystals ofInGaZnO₄ exist in the whole crystal region 109 is more effective andmore preferable.

As described above, the oxide semiconductor film 108 has the crystalregion 109 in the superficial portion, whereby favorable electricalcharacteristics can be achieved. In particular, in the case where thecrystal region 109 includes InGaZnO₄ crystals c-axes of which areoriented in a direction almost perpendicular to the top surface of theoxide semiconductor film 108, the carrier mobility in the superficialportion of the oxide semiconductor film 108 is increased by theelectrical characteristics of the InGaZnO₄ crystals. Thus, thefield-effect mobility of the transistor in which the oxide semiconductorfilm 108 is included is increased, which leads to favorable electricalcharacteristics of the transistor.

Further, the crystal region 109 is more stable than the amorphous region110; therefore, when the crystal region 109 is included in thesuperficial portion of the oxide semiconductor film 108, the entry ofimpurities (e.g., hydrogen, water, a hydroxy group, hydride, or thelike) into the amorphous region 110 can be suppressed. Thus, thereliability of the oxide semiconductor film 108 can be improved.

Through the above-described process, the concentration of hydrogen inthe oxide semiconductor film can be reduced and the oxide semiconductorfilm can be highly purified. Thus, the oxide semiconductor film can bestabilized. In addition, heat treatment at a temperature which is lowerthan or equal to the glass transition temperature makes it possible toform an oxide semiconductor film with a wide band gap in which carrierdensity is extremely low. Therefore, a transistor can be manufacturedusing a large-sized substrate, so that mass productivity can beincreased. In addition, by using the oxide semiconductor film whosehydrogen concentration is reduced and which is highly purified, it ispossible to form a transistor with a high withstand voltage, lessshort-channel effect, and a high on/off ratio.

The amorphous region 110 is mainly an amorphous oxide semiconductorfilm. The word “mainly” means, for example, a state where the occupancyis 50% or more, and means a state where the amorphous region 110 isoccupied by the amorphous oxide semiconductor film at an occupancy of50% or more by volume (or weight) in this case. In other words, theamorphous region in some cases includes crystals of an oxidesemiconductor film other than an amorphous oxide semiconductor film, andthe occupancy thereof is preferably less than 50% by volume (or weight).However, the occupancy is not limited to the above.

In the case where the In—Ga—Zn—O-based oxide semiconductor film is usedas a material of the oxide semiconductor film, the composition of theabove-described amorphous region 110 is preferably set so that a Zncontent (atomic %) is larger than an In or Ga content (atomic %). Such acomposition makes it easy to form the crystal region 109 with apredetermined composition.

Although the manufacturing method in which the oxide semiconductor film103 is processed into a predetermined shape to form the island-shapedoxide semiconductor film 104, and after that, the first heat treatment,the addition of oxygen, and the second heat treatment are performed isdescribed in Embodiment 1, the present invention is not limited to thisstructure. The first heat treatment, the addition of oxygen, and thesecond heat treatment may be performed on the oxide semiconductor film103 before being processed into the island-shaped oxide semiconductorfilm 104, and after that, the oxide semiconductor film may be processedinto a predetermined shape to form the island-shaped oxide semiconductorfilm 104. Alternatively, the first heat treatment may be performed onthe oxide semiconductor film 103, and after that, the oxidesemiconductor film may be processed into a predetermined shape to formthe island-shaped oxide semiconductor film, and then, the addition ofoxygen and the second heat treatment may be performed on theisland-shaped oxide semiconductor film. Further alternatively, the firstheat treatment and the addition of oxygen may be performed on the oxidesemiconductor film 103, and after that, the oxide semiconductor film maybe processed into a predetermined shape to form the island-shaped oxidesemiconductor film, and then, the second heat treatment may be performedon the island-shaped oxide semiconductor film.

Next, as shown in FIG. 2A, a conductive film which forms a sourceelectrode and a drain electrode (including a wiring formed from the samelayer as the source electrode or the drain electrode) is formed over thegate insulating film 102 and the oxide semiconductor film 108 and ispatterned to form the source electrode 111 and the drain electrode 112.The conductive film may be formed by a sputtering method or a vacuumevaporation method. As a material of the conductive film to be thesource and drain electrodes (including the wiring formed from the samelayer as the source or the drain electrode), there are an elementselected from Al, Cr, Cu, Ta, Ti, Mo, or W; an alloy including any ofthe above elements; an alloy film containing a combination of any ofthese elements; and the like. A structure may be employed in which afilm of a high-melting point metal such as Cr, Ta, Ti, Mo, or W and ametal film of Al, Cu, or the like are stacked. By using an Al materialto which an element which prevents generation of hillocks or whisker inan Al film, such as Si, Ti, Ta, W, Mo, Cr, Nd, Sc, or Y is added, heatresistance can be improved.

The conductive film may have a single-layer structure or a stacked-layerstructure of two or more layers. For example, a single-layer structureof an aluminum film including silicon, a two-layer structure in which atitanium film is stacked over an aluminum film, a three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order, and the like can be given.

Alternatively, the conductive film to be the source and drain electrodes(including the wiring formed from the same layer as the source or thedrain electrode) may be formed using a conductive metal oxide. As aconductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), an alloy of indium oxide and tin oxide (In₂O₃—SnO₂,abbreviated to ITO), an alloy of indium oxide and zinc oxide(In₂O₃—ZnO), or the metal oxide material to which silicon or siliconoxide is added can be used.

In the case where heat treatment is performed after the deposition ofthe conductive film, it is preferable that the conductive film have heatresistance high enough to withstand the heat treatment.

Then, a resist mask is formed over the conductive film, and selectiveetching is performed, so that the source electrode 111 and the drainelectrode 112 are formed. After that, the resist mask is removed.

Ultraviolet, a KrF laser beam, or an ArF laser beam is used for lightexposure for forming the resist mask in a photolithography step. Thechannel length L of a transistor to be formed later is determined by apitch between a lower end of the source electrode and a lower end of thedrain electrode that are adjacent to each other over the oxidesemiconductor film 108. Note that when light exposure is performed inthe case where the channel length L is shorter than 25 nm, extremeultraviolet with extremely short wavelengths of several nanometers toseveral tens of nanometers is used for light exposure for forming theresist mask in the photolithography step. Light exposure with extremeultraviolet leads to a high resolution and a large depth of focus. Thus,the channel length L of the transistor can be greater than or equal to10 nm and less than or equal to 1000 nm and the operation speed of acircuit can be increased and furthermore the value of off-state currentis extremely small, so that low power consumption can be achieved.

Each material and etching conditions are adjusted as appropriate so thatthe oxide semiconductor film 108 is not removed in etching of theconductive film as much as possible.

In this embodiment, a titanium film is used as the conductive film, asolution (an ammonia hydrogen peroxide mixture) containing ammonia andhydrogen peroxide is used, and the conductive film is wet-etched,whereby the source electrode 111 and the drain electrode 112 are formed.As the ammonia hydrogen peroxide mixture, specifically, a solution inwhich oxygenated water of 31 wt %, ammonia water of 28 wt %, and waterare mixed at a volume ratio of 5:2:2 is used. Alternatively, dry etchingmay be performed on the conductive film 105 with the use of a gascontaining chlorine (Cl₂), boron chloride (BCl₃), or the like.

In the patterning for forming the source electrode 111 and the drainelectrode 112, an exposed portion of the island-shaped oxidesemiconductor film 108 may be partly etched so that a groove (a recessedportion) is formed in the island-shaped oxide semiconductor film 108.The resist mask used for forming the source electrode 111 and the drainelectrode 112 may be formed by an inkjet method. Formation of the resistmask by an inkjet method needs no photomask; thus, manufacturing costscan be reduced.

In order to reduce the number of photomasks and steps in aphotolithography step, etching may be performed with the use of a resistmask formed using a multi-tone mask which is a light-exposure maskthrough which light is transmitted so as to have a plurality ofintensities. Such a resist mask formed with the use of a multi-tone maskhas a plurality of thicknesses and further can be changed in shape byetching; therefore, the resist mask can be used in a plurality ofetching steps for processing into different patterns. Therefore, aresist mask corresponding to at least two kinds of different patternscan be formed by one multi-tone mask. Thus, the number of light-exposuremasks can be reduced and the number of corresponding photolithographysteps can be also reduced, whereby simplification of a process can berealized.

The source electrode 111 and the drain electrode 112 are in contact withthe crystal region 109 of the oxide semiconductor film 108. Owing tothis contact between the highly conductive crystal region 109 and eachof the source electrode 111 and the drain electrode 112, the contactresistance between each of the source electrode 111 and the drainelectrode 112 and the oxide semiconductor film 108 can be reduced, sothat the on-state current of the transistor formed can be increased.

Next, plasma treatment is performed thereon, using a gas such as N₂O,N₂, or Ar. By the plasma treatment, adsorbed water or the like whichattaches to an exposed surface of the oxide semiconductor film isremoved. Plasma treatment may be performed using a mixture gas of oxygenand argon as well.

After the plasma treatment, as shown in FIG. 2B, an insulating film 113is formed to cover the source electrode 111, the drain electrode 112,and the oxide semiconductor film 108. The insulating film 113 preferablyincludes no impurities such as moisture or hydrogen as much as possible,and may be formed using a single-layer insulating film or a plurality ofinsulating films stacked. When hydrogen is contained in the insulatingfilm 113, entry of hydrogen to the oxide semiconductor film orextraction of oxygen contained in the oxide semiconductor film byhydrogen is caused; thus, a backchannel portion of the oxidesemiconductor film might have low resistance (n-type conductivity) and aparasitic channel might be formed. Therefore, it is preferable that aformation method in which hydrogen is not used is employed in order toform the insulating film 113 containing as little hydrogen as possible.A material having a high barrier property is preferably used for theinsulating film 113. For example, as the insulating film having a highbarrier property, a silicon nitride film, a silicon nitride oxide film,an aluminum nitride film, an aluminum nitride oxide film, or the likecan be used. When a plurality of insulating films stacked is used, aninsulating film having lower proportion of nitrogen than the insulatingfilm having a high barrier property, such as a silicon oxide film or asilicon oxynitride film, is formed on the side close to the oxidesemiconductor film 108. Then, the insulating film having a barrierproperty is formed so as to overlap with the source electrode 111, thedrain electrode 112, and the oxide semiconductor film 108 with theinsulating film having lower proportion of nitrogen providedtherebetween. With the insulating film having a barrier property,impurities such as moisture or hydrogen can be prevented from enteringthe oxide semiconductor film 108, the gate insulating film 102, or theinterface between the oxide semiconductor film 108 and anotherinsulating film and the vicinity thereof. In addition, by forming theinsulating film having lower proportion of nitrogen such as a siliconoxide film or a silicon oxynitride film so as to be in contact with theoxide semiconductor film 108, the insulating film formed using amaterial having a high barrier property can be prevented from being incontact with the oxide semiconductor film 108 directly.

In this embodiment, the insulating film 113 has a structure in which asilicon nitride film having a thickness of 100 nm formed by a sputteringmethod is stacked over a silicon oxide film having a thickness of 200 nmformed by a sputtering method. The substrate temperature in the filmformation may be higher than or equal to room temperature and lower thanor equal to 300° C. and is 100° C. in this embodiment.

After the insulating film 113 is formed, heat treatment may beperformed. The heat treatment is performed in an inert gas atmosphere(nitrogen, helium, neon, argon, or the like) at a temperature,preferably, higher than or equal to 200° C. and lower than or equal to400° C., for example, at a temperature higher than or equal to 250° C.and lower than or equal to 350° C. In this embodiment, heat treatmentfor 1 hour at 250° C. in a nitrogen atmosphere is performed.Alternatively, an RTA treatment for a short time at a high temperaturemay be performed before the formation of the source electrode 111 andthe drain electrode 112 in a similar manner to the heat treatmentperformed on the oxide semiconductor film. By the heat treatmentperformed on the state where the insulating film 113 containing oxygenis in contact with an exposed region of the oxide semiconductor film108, provided between the source electrode 111 and the drain electrode112, oxygen is supplied to the oxide semiconductor film 108, whereby theregion of the oxide semiconductor film 108, which is in contact with theinsulating film 113 can be selectively made an oxygen-excess state.Consequently, a structure which satisfies the stoichiometric compositionratio can be obtained, and a channel formation region which overlapswith the gate electrode 101 becomes i-type, which leads to improvementof the electrical characteristics of the transistor and suppression ofvariation of the electrical characteristics. The timing of this heattreatment is not particularly limited as long as it is after theformation of the insulating film 113, and can be performed withoutincreasing the number of manufacturing steps by doubling as another stepsuch as a heat treatment for a formation of a resin film or a heattreatment for reduction of the resistance of a transparent conductivefilm.

Through the above-described process, a transistor 114 is formed.

FIG. 2C is a top view of the transistor 114 shown in FIG. 2B. Across-sectional view along dashed line A1-A2 in FIG. 2C corresponds toFIG. 2B.

The transistor 114 includes the gate electrode 101 formed over thesubstrate 100 having an insulating surface, the gate insulating film 102over the gate electrode 101, the oxide semiconductor film 108 whichoverlaps with the gate electrode 101 over the gate insulating film 102,and a pair of the source electrode 111 and the drain electrode 112formed over the oxide semiconductor film 108. The transistor 114 mayinclude the insulating film 113 provided over the oxide semiconductorfilm 108. The transistor 114 shown in FIG. 2C has a channel-etchedstructure in which part of the oxide semiconductor film 108 is etchedbetween the source electrode 111 and the drain electrode 112.

Although the transistor 114 is described as a single-gate transistor inEmbodiment 1, a multi-gate transistor including a plurality of channelformation regions can be formed when needed.

The transistor 114 formed by the manufacturing method shown in FIGS. 1Ato 1D and 2A to 2C has a structure in which part of the crystal region109, which is provided between the source electrode 111 and the drainelectrode 112 is removed by etching to expose the amorphous region 110.However, whether the amorphous region 110 is exposed or not depends onthe depth to which the superficial portion where the crystal region 109exists reaches from the top surface of the oxide semiconductor film 108and the amount by which the top surface of the oxide semiconductor film108 is etched in forming the source electrode 111 and the drainelectrode 112.

FIG. 11A is a cross-sectional view of the oxide semiconductor film 108in the case where the oxide semiconductor film 108 includes the crystalregion 109 and the amorphous region 110 and the superficial portionwhere the crystal region 109 exists reaches to a distance (depth) fromthe top surface of one half or more of the thickness of the oxidesemiconductor film 108. In addition, FIG. 11B illustrates an example ofa cross-sectional view of a channel-etched transistor using the oxidesemiconductor film 108 shown in FIG. 11A. In FIG. 11B, the superficialportion where the crystal region 109 exists reaches to a larger depthfrom the top surface than that of the transistor 114 shown in shown inFIGS. 1A to 1D and 2A to 2C, and therefore, part of the crystal region109 between the source electrode 111 and the drain electrode 112 isleft.

One embodiment of the present invention may have a structure in whichthe amorphous region 110 is exposed between the source electrode 111 andthe drain electrode 112 as shown in FIG. 2B or a structure in which thecrystal region 109 is left as shown in FIG. 11B. However, in the case ofa channel-etched transistor having a bottom-gate structure, in order toprevent formation of a parasitic channel in a back-channel portion ofthe oxide semiconductor film 108, which is far from the gate electrode101, it is preferable that the back-channel portion is formed from theamorphous region 110 which has high resistance. Therefore, the on/offratio of the transistor can be increased by employing the structure inwhich the amorphous region 110 is exposed between the source electrode111 and the drain electrode 112 as shown in FIG. 2B rather than thestructure as shown in FIG. 11B.

In accordance with further progress of crystallization of the oxidesemiconductor film 108, the oxide semiconductor film 108 may be almostwholly occupied by the crystal region 109. FIG. 12A is a cross-sectionalview of the oxide semiconductor film 108 in the case where the oxidesemiconductor film 108 is almost wholly occupied by the crystal region109. FIG. 12B illustrates an example of a cross-sectional view of achannel-etched transistor using the oxide semiconductor film 108 shownin FIG. 12A. In FIG. 12B, a region of the oxide semiconductor film 108,which overlaps with the gate electrode 101, that is, a channel formationregion, is wholly formed from the crystal region 109. By employing theabove-described structure, the carrier mobility in the channel formationregion is increased, so that the field-effect mobility of the transistoris increased and favorable electrical characteristics can be achieved.

Next, a conductive film may be formed over the insulating film 113 andmay be patterned so that a back gate electrode 115 is formed so as tooverlap with the oxide semiconductor film 108 as shown in FIG. 3A. Theback gate electrode 115 can be formed using a material and a structurewhich are similar to those of the gate electrode 101 or the sourceelectrode 111 or the drain electrode 112.

The thickness of the back gate electrode 115 is set to 10 nm to 400 nm,preferably 100 nm to 200 nm. In this embodiment, a conductive film inwhich a titanium film, an aluminum film, and a titanium film are stackedis formed. Then, a resist mask is formed by a photolithography method,and an unnecessary portion is removed by etching so that the conductivefilm is processed (patterned) into an appropriate shape; thus, the backgate electrode 115 is formed.

Next, as shown in FIG. 3B, an insulating film 116 is formed so as tocover the back gate electrode 115. The insulating film 116 is preferablyformed using a material with a high barrier property that can preventmoisture, hydrogen, oxygen, and the like in an atmosphere from affectingthe characteristics of the transistor 114. For example, the insulatingfilm having a high barrier property can be formed to have a single-layerstructure or a stacked-layer structure of a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, or the like by a plasma CVD method, a sputteringmethod, or the like. In order to obtain an effect of a barrier property,the insulating film 116 is preferably formed to a thickness of 15 nm to400 nm, for example.

In this embodiment, an insulating film with a thickness of 300 nm isformed by a plasma CVD method. The insulating film is formed under thefollowing conditions: the flow rate of a silane gas is 4 sccm; the flowrate of dinitrogen monoxide (N₂O) is 800 sccm; and the substratetemperature is 400° C.

A top view of the semiconductor device shown in FIG. 3B is FIG. 3C. FIG.3B is a cross-sectional view along dashed line A1-A2 in FIG. 3C.

Although the back gate electrode 115 covers the oxide semiconductor film108 entirely in FIG. 3B, one embodiment of the present invention is notlimited to this structure. The back gate electrode 115 overlaps with atleast part of the channel formation region included in the oxidesemiconductor film 108.

The back gate electrode 115 may be electrically insulated to be in afloating state, or may be in a state where the back gate electrode 115is supplied with a potential. In the latter case, to the back gateelectrode 115, a potential which is the same level as the gate electrode101 may be applied, or a fixed potential such as ground may be applied.The level of the potential supplied to the back gate electrode 115 iscontrolled, whereby the threshold voltage of the transistor 114 can becontrolled.

How characteristics of the transistor are influenced by highpurification of the oxide semiconductor film by removal of impuritiessuch as hydrogen, water, or the like contained in the oxidesemiconductor film as in this embodiment is described below.

FIG. 22 is a cross-sectional view of an inverted staggered transistorincluding an oxide semiconductor. An oxide semiconductor film (OS) isprovided over a gate electrode (GE) with a gate insulating film (GI)provided therebetween, and a source electrode (S) and a drain electrode(D) are provided thereover.

FIG. 23 is an energy band diagram (schematic diagram) along section A-A′in FIG. 22 . In FIG. 23 , a black circle (●) and a white circle (◯)represent an electron and a hole and have electric charges (−q, +q),respectively. With a positive voltage (VD>0) applied to the drainelectrode, the dashed line shows the case where no voltage is applied tothe gate electrode (VG=0) and the solid line shows the case where apositive voltage is applied to the gate electrode (VG>0). In the casewhere no voltage is applied to the gate electrode, carriers (electrons)are not injected to the oxide semiconductor side from an electrodebecause of high potential barrier, so that a current does not flow,which means an off state. On the other hand, when a positive voltage isapplied to the gate electrode, potential barrier is lowered, and thus acurrent flows, which means an on state.

FIGS. 24A and 24B are energy band diagrams (schematic diagrams) alongsection B-B′ in FIG. 22 . FIG. 24A illustrates the state where apositive voltage (VG>0) is applied to the gate electrode (GE) and an onstate where carriers (electrons) flow between the source electrode andthe drain electrode. FIG. 24B illustrates the state where a negativevoltage (VG<0) is applied to the gate electrode (GE) and an off state (aminority carrier does not flow).

FIG. 25 illustrates the relation between the vacuum level, the workfunction of metal (φ_(M)), and the electron affinity of an oxidesemiconductor (χ).

At normal temperature, electrons in the metal are degenerated and theFermi level is located in the conduction band. On the other hand, ingeneral, a conventional oxide semiconductor is an n-type semiconductor,and the Fermi level (Ef) thereof is located near the conduction band(Ec) away from an intrinsic Fermi level (Ei) which is located in thecenter of the band gap. It is known that part of hydrogen in the oxidesemiconductor becomes a donor and one of factors that make the oxidesemiconductor an n-type semiconductor. Further, an oxygen defect isknown as one of factors that make the conductivity type an n-type.

According to one embodiment of the present invention, hydrogen that isan n-type impurity is removed from an oxide semiconductor to highlypurify the oxide semiconductor so that impurities that are not maincomponents of the oxide semiconductor are not contained as much aspossible and to remove an oxygen defect, whereby an i-type (intrinsic)or substantially intrinsic oxide semiconductor is obtained. That is, anoxide semiconductor is made to be an oxide semiconductor which is ani-type (intrinsic) semiconductor or is a substantially i-type(intrinsic) semiconductor not by adding an impurity but by removingimpurities such as hydrogen or water or an oxygen defect as much aspossible to have high purity. With the above-described structure, theFermi level (Ef) can be substantially close to the same level as theintrinsic Fermi level (Ei), as indicated by arrows.

The band gap (Eg) of an oxide semiconductor is said to be 3.15 eV, andthe electron affinity (χ) is said to be 4.3 eV. The work function oftitanium (Ti) included in the source electrode and the drain electrodeis substantially equal to the electron affinity (χ) of the oxidesemiconductor. In that case, a Schottky barrier to electrons is notformed at an interface between the metal and the oxide semiconductor.

In that case, as shown in FIG. 24A, the electron moves along the lowestpart of the oxide semiconductor, which is energetically stable, at aninterface between the gate insulating film and the highly-purified oxidesemiconductor.

In FIG. 24B, when a negative potential (reverse bias) is applied to thegate electrode (GE), holes which are minority carriers are substantiallyzero; therefore, current is substantially close to zero.

In such a manner, an intrinsic (i-type) or substantially intrinsic oxidesemiconductor is obtained by being purified such that an element otherthan its main element (i.e., an impurity element) is contained as littleas possible. Thus, characteristics of the interface between the oxidesemiconductor and the gate insulating layer become obvious. For thatreason, the gate insulating layer needs to be able to form a favorableinterface with the oxide semiconductor. Specifically, it is preferableto use, for example, an insulating layer formed by a CVD method usinghigh-density plasma generated with a power supply frequency in the rangeof the VHF band to the microwave band, an insulating layer formed by asputtering method, or the like.

The oxide semiconductor is purified and the interface between the oxidesemiconductor and the gate insulating layer is made favorable.

For example, even when the transistor has a channel width W of 1×10⁴ μmand a channel length L of 3 μm, an off-state current of 10⁻¹³ A or lessand a subthreshold swing (S value) of 0.1 V/dec. (the thickness of thegate insulating film: 100 nm) can be obtained.

In this manner, the oxide semiconductor film is highly purified so thatimpurities such as water or hydrogen except a main component of theoxide semiconductor are not contained as much as possible, wherebyfavorable operation of the transistor can be obtained.

Embodiment 2

In Embodiment 2, a structure and a manufacturing method of a transistorfor a power device capable of higher voltage or higher current controlwill be described. Embodiment 1 can be applied to the same portions asEmbodiment 1 or portions having functions similar to those of Embodiment1, and therefore, description thereof is not omitted.

As shown in FIG. 5A, an insulating film 201 which serves as a base filmis formed over a substrate 200, and then, a first electrode 202 isformed.

The description on the substrate 100 described in Embodiment 1 can bereferred to for a substrate to be used as the substrate 200. Thedescription on the base film described in Embodiment 1 can be referredto for a material, the structure, and the thickness of the insulatingfilm 201.

The first electrode 202 is formed using a metal element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, tungsten, oryttrium; an alloy containing any of these metal elements as a component;an alloy containing these metal elements in combination; or the like.Alternatively, one or more material elements selected from manganese,magnesium, zirconium, beryllium, and thorium can be used. In addition,the first electrode 202 can have a single-layer structure or astacked-layer structure having two or more layers. For example, asingle-layer structure of an aluminum film containing silicon; atwo-layer structure of an aluminum film and a titanium film stackedthereover; a two-layer structure of a tungsten film and a titanium filmstacked thereover; a three-layer structure in which a titanium film, analuminum film, and a titanium film are stacked in that order; and thelike can be given. Alternatively, a film, an alloy film, or a nitridefilm which contains aluminum and one or more elements selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

Alternatively, a light-transmitting conductive material such as indiumtin oxide, indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide to which silicon oxide is added, can be used as the firstelectrode 202. A stacked-layer structure of the above-describedlight-transmitting conductive material and the above-described metalelement may be employed.

The first electrode 202 can be formed in such a manner that a conductivefilm is formed over the substrate 200 by a sputtering method, a CVDmethod, or a vacuum evaporation method, a resist mask is formed over theconductive film in a photolithography step, and the conductive film isetched using the resist mask. Alternatively, the first electrode 202 canbe formed by a printing method or an inkjet method without using aphotolithography step, so that the number of steps can be reduced. Notethat end portions of the first electrode 202 preferably have a taperedshape, so that the coverage with a gate insulating film formed laterimproves. When the angle between the end portion of the first electrode202 and the insulating film 201 is greater than or equal to 30° and lessthan or equal to 60°, preferably greater than or equal to 40° and lessthan or equal to 50°, the coverage with the gate insulating film formedlater can be improved.

In this embodiment, as the conductive film serving as the firstelectrode 202, a 50-nm-thick titanium film is formed by a sputteringmethod, a 100-nm-thick aluminum film is formed, and a 50-nm-thicktitanium film is formed. Next, the conductive film is etched with theuse of a resist mask formed by a photolithography step, so that thefirst electrode 202 is formed. Instead of the resist mask formed by thephotolithography step, a resist mask may be formed using an ink-jetmethod, so that the number of steps can be reduced.

Next, an island-shaped oxide semiconductor film 203 is formed over thefirst electrode 202. The oxide semiconductor film 203 can be formed by asputtering method, a coating method, a printing method, or the like. Inthis embodiment, the island-shaped oxide semiconductor film 203 isformed as follows: an oxide semiconductor film is formed over the firstelectrode 202 by a sputtering method and is processed into anappropriate shape by etching or the like. The oxide semiconductor filmcan be formed by a sputtering method under a rare gas (for example,argon) atmosphere, an oxygen atmosphere, or an atmosphere including arare gas (for example, argon) and oxygen.

The etching for forming the island-shaped oxide semiconductor film 203may be performed according to the description on the etching for formingthe island-shaped oxide semiconductor film 203 described inEmbodiment 1. The angle formed between the first electrode 202 and anend portion of the island-shaped oxide semiconductor film 203 formed bythe etching is set to greater than or equal to 30° and less than orequal to 60°, preferably greater than or equal to 40° and less than orequal to 50°, which is preferable because the coverage with the gateinsulating film formed later can be improved.

Note that before the oxide semiconductor film is formed by a sputteringmethod, reverse sputtering in which an argon gas is introduced andplasma is generated is preferably performed to remove dust attached to asurface of the first electrode 202. The reverse sputtering refers to amethod in which, without application of a voltage to a target side, anRF power source is used for voltage application to a substrate side inan argon atmosphere to modify a surface. Instead of an argon atmosphere,a nitrogen atmosphere, a helium atmosphere, or the like may be used.Alternatively, an argon atmosphere to which oxygen, nitrous oxide, orthe like is added may be used. Further alternatively, an argonatmosphere to which chlorine, carbon tetrafluoride, or the like is addedmay be used.

The above-described oxide semiconductor can be used for the oxidesemiconductor film 203.

In this embodiment, as the oxide semiconductor film 203, anIn—Ga—Zn—O-based non-single-crystal film with a thickness of 30 nm,which is obtained by a sputtering method using an oxide semiconductortarget including indium (In), gallium (Ga), and zinc (Zn) is used. Asthe target, for example, an oxide semiconductor target having acomposition ratio with an atom ratio of metals, In:Ga:Zn=1:1:0.5,In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used. Further, the oxidesemiconductor film can be formed by a sputtering method under a rare gas(typically argon) atmosphere, an oxygen atmosphere, or an atmospherecontaining a rare gas (typically argon) and oxygen. In the case of usinga sputtering method, a target containing SiO₂ at 2 wt % to 10 wt % bothinclusive may be used for depositing the film. The filling rate of theoxide semiconductor target including In, Ga, and Zn is greater than orequal to 90% and less than or equal to 100%, preferably greater than orequal to 95% and less than or equal to 99.9%. With the use of the oxidesemiconductor target with a high filling rate, a dense oxidesemiconductor film is formed.

The oxide semiconductor film 203 is formed over the substrate 200 insuch a manner that the substrate is held in the treatment chambermaintained at reduced pressure, a sputtering gas from which hydrogen andmoisture have been removed is introduced into the treatment chamberwhile moisture remaining therein is removed, and metal oxide is used asa target. At that time, the substrate may be heated at higher than orequal to 100° C. and lower than or equal to 600° C., preferably higherthan or equal to 200° C. and lower than or equal to 400° C. Filmdeposition may be performed while the substrate is heated, whereby theconcentration of an impurity contained in the oxide semiconductor filmdeposited can be reduced. In addition, damage by sputtering can bereduced. In order to remove remaining moisture in the treatment chamber,an entrapment vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. Anevacuation unit may be a turbo pump provided with a cold trap. In thedeposition chamber which is evacuated with the cryopump, for example, ahydrogen atom, a compound containing a hydrogen atom, such as water(H₂O), (far preferably, also a compound containing a carbon atom), andthe like are removed, whereby the concentration of an impurity in theoxide semiconductor film deposited in the deposition chamber can bereduced.

As one example of the deposition condition, the following can beemployed: the substrate temperature is room temperature, the distancebetween the substrate and the target is 110 mm, the pressure is 0.4 Pa,the direct-current (DC) power is 0.5 kW, and the atmosphere is anatmosphere of oxygen and argon (the oxygen flow rate 15 sccm:the argonflow rate 30 sccm). Note that a pulsed direct-current (DC) power sourceis preferable because powder substances which are referred to asparticles generated in film deposition can be reduced and the filmthickness can be uniform. The oxide semiconductor film has a thicknessgreater than or equal to 1 μm, preferably greater than or equal to 3 μm,far preferably 10 μm. Since appropriate thickness depends on an oxidesemiconductor material used, the thickness can be determined asappropriate depending on the material.

Further, in order that hydrogen, a hydroxyl group, and moisture may becontained in the oxide semiconductor film 203 as little as possible, itis preferable that the substrate 200 on which the process up to andincluding the step of forming the first electrode 202 is alreadyperformed be preheated in a preheating chamber of a sputtering apparatusas pretreatment for film formation so that impurities such as hydrogenand moisture adsorbed to the substrate 200 are removed and exhausted.The temperature of the preheating is higher than or equal to 100° C. andlower than or equal to 400° C., preferably higher than or equal to 150°C. and lower than or equal to 300° C. As an exhaustion unit provided inthe preheating chamber, a cryopump is preferable. This preheatingtreatment can be omitted. Further, this preheating may be similarlyperformed on the substrate 200 on which the process up to and includingthe step of forming a gate electrode is already performed, before theformation of a gate insulating film.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power supply, aDC sputtering method, and a pulsed DC sputtering method in which a biasis applied in a pulsed manner. An RF sputtering method is mainly used inthe case where an insulating film is formed, and a DC sputtering methodis mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

Alternatively, a sputtering apparatus provided with a magnet systeminside the chamber and used for a magnetron sputtering method, or asputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge can be used.

Further, as a deposition method using a sputtering method, a reactivesputtering method in which a target substance and a sputtering gascomponent are chemically reacted with each other during deposition toform a thin compound film thereof, or a bias sputtering method in whicha voltage is also applied to a substrate during deposition can be used.

Next, first heat treatment is performed on the oxide semiconductor film203 in a reduced-pressure atmosphere, an inert gas atmosphere such as anitrogen atmosphere or a rare gas atmosphere, an oxygen gas atmosphere,or an ultra dry air atmosphere (in air whose moisture content is lessthan or equal to 20 ppm (dew point conversion, —55° C.), preferably lessthan or equal to 1 ppm, far preferably less than or equal to 10 ppb inthe case where measurement is performed using a dew-point meter of acavity ring-down laser spectroscopy (CRDS) system). By the first heattreatment on the oxide semiconductor film 203, an oxide semiconductorfilm 205 in which moisture or hydrogen is eliminated is formed as shownin FIG. 5B. Specifically, heat treatment may be performed at atemperature higher than or equal to 500° C. and lower than or equal to850° C. (or a temperature lower than or equal to a strain point of aglass substrate), preferably a temperature higher than or equal to 550°C. and lower than or equal to 750° C. For example, heat treatment may beperformed at 600° C. for a period longer than or equal to 3 minutes andshorter than or equal to 6 minutes. Since dehydration or dehydrogenationcan be performed in a short time with the RTA method, the first heattreatment can be performed even at a temperature over the strain pointof a glass substrate. In this embodiment, heat treatment is performed onthe oxide semiconductor film 203 at a substrate temperature of 600° C.for 6 minutes in a nitrogen atmosphere with the use of an electricalfurnace that is one of heat treatment apparatuses, and then, the oxidesemiconductor film is not exposed to the air and water and hydrogen areprevented from entering the oxide semiconductor film, so that the oxidesemiconductor film 205 is obtained.

Detailed description on the heat treatment apparatus used for the firstheat treatment is already made in Embodiment 1 and therefore omittedhere.

It is preferable that in the heat treatment, moisture, hydrogen, or thelike be not contained in nitrogen or a rare gas such as helium, neon, orargon. The purity of nitrogen or the rare gas such as helium, neon, orargon which is introduced into the heat treatment apparatus ispreferably set to be 6N (99.9999%) or higher, far preferably 7N(99.99999%) or higher (that is, the impurity concentration is preferably1 ppm or lower, far preferably 0.1 ppm or lower).

Thus, as shown in FIG. 5B, by the first heat treatment, a superficialportion of the island-shaped oxide semiconductor film 205 is made tohave a crystal region 206. The crystal region 206 includes a so-callednanocrystal with a grain size greater than or equal to 1 nm and lessthan or equal to 20 nm, and the other portion of the island-shaped oxidesemiconductor film 205 other than the crystal region 206 is amorphous orincludes a mixture of an amorphous state and microcrystals where themicrocrystals are provided in the amorphous state. Note that theabove-described size of the nanocrystal is just an example, and thepresent invention is not construed as being limited to the above range.In the case where the oxide semiconductor film is an In—Ga—Zn—O-basedoxide semiconductor film formed by a sputtering method using a targetthe atom ratio of metals of which is In:Ga:Zn=1:1:1, crystallization ofthe superficial portion of the oxide semiconductor film is likely to befurther promoted as compared to the case where a target having adifferent atom ratio is used, so that the crystal region 206 is likelyto be formed to a larger depth.

Next, as shown in FIG. 5C, oxygen is added into the oxide semiconductorfilm 205 having the crystal region 206 in the superficial portion by anion implantation method or an ion doping method. Oxygen is added intothe oxide semiconductor film 205 by an ion implantation method, an iondoping method, or the like, so that an oxide semiconductor film 207 inwhich oxygen is added excessively is formed. By adding oxygen, a bondbetween a metal as a component of the oxide semiconductor and hydrogenor a bond between the metal and a hydroxyl group is cut and the hydrogenor the hydroxyl group is reacted with oxygen to produce water; thisleads to easy elimination of hydrogen or a hydroxyl group that is animpurity, in the form of water by second heat treatment performed later.

In the case where an oxygen gas is used and oxygen is added by an ionimplantation method, the acceleration voltage may be set in the range of5 kV to 100 kV both inclusive and the dosage may be set in the range of1×10¹³ ions/cm² to 1×10¹⁶ ions/cm² both inclusive.

Heat treatment may be performed on the substrate provided with the oxidesemiconductor film 205 at a temperature higher than or equal to 500° C.and lower than or equal to 850° C. (or a temperature lower than or equalto a strain point of a glass substrate), preferably a temperature higherthan or equal to 550° C. and lower than or equal to 750° C. while theaddition of oxygen into the oxide semiconductor film 205 is performed byan ion implantation method.

Crystals included in the crystal region 206 formed in the superficialportion of the oxide semiconductor film 205 are damaged by addition ofoxygen using an ion implantation method, an ion doping method, or thelike. Therefore, the crystallinity of a superficial portion of the oxidesemiconductor film 207 is lower than that of the crystal region 206 ofthe oxide semiconductor film 205 before the oxygen addition, and thesuperficial portion of the oxide semiconductor film 207 may be in asimilar state to the amorphous region of the oxide semiconductor film205 depending on the dosage of oxygen.

Next, second heat treatment is performed. The second heat treatment canbe performed in a similar condition to the first heat treatment.Specifically, heat treatment may be performed in a reduced-pressureatmosphere, an inert gas atmosphere such as a nitrogen atmosphere or arare gas atmosphere, an oxygen gas atmosphere, or an ultra dry airatmosphere (in air whose moisture content is less than or equal to 20ppm (dew point conversion, —55° C.), preferably less than or equal to 1ppm, far preferably less than or equal to 10 ppb in the case wheremeasurement is performed using a dew-point meter of a cavity ring-downlaser spectroscopy (CRDS) system) at a temperature higher than or equalto 500° C. and lower than or equal to 850° C. (or a temperature lowerthan or equal to a strain point of a glass substrate), preferably atemperature higher than or equal to 550° C. and lower than or equal to750° C. In the case where heat treatment is performed by RTA (RapidThermal Anneal), for example, heat treatment may be performed at 600° C.for a period longer than or equal to 3 minutes and shorter than or equalto 6 minutes. Since dehydration or dehydrogenation can be performed in ashort time with the RTA method, the second heat treatment can beperformed even at a temperature over the strain point of a glasssubstrate. In this embodiment, heat treatment is performed at asubstrate temperature of 600° C. for 6 minutes in a nitrogen atmospherewith the use of an electrical furnace that is one of heat treatmentapparatuses, and then, the oxide semiconductor film is not exposed tothe air and water and hydrogen are prevented from entering the oxidesemiconductor film, so that an oxide semiconductor film 208 is obtained.The heat treatment may be performed plural times after the island-shapedoxide semiconductor film 208 is formed.

According to one embodiment of the present invention, the bond between ametal as a component of the oxide semiconductor and hydrogen or ahydroxyl group is cut and the hydrogen or the hydroxyl group is made toreact with oxygen to produce water by adding oxygen in the oxidesemiconductor film 205. Thus, an impurity such as hydrogen or a hydroxylgroup left in the film can be easily eliminated in the form of water bythe second heat treatment after the oxygen addition. The island-shapedoxide semiconductor film 208 formed through the second heat treatment ismore i-type (intrinsic) or closer to i-type than the oxide semiconductorfilm 205 after the first heat treatment because impurities such asmoisture or hydrogen left even after the first heat treatment areremoved. Impurities such as moisture or hydrogen are eliminated, and theisland-shaped oxide semiconductor film becomes an i-type (intrinsic)semiconductor or a substantially i-type semiconductor; therefore,deterioration of characteristics of the transistor due to theimpurities, such as shifts in threshold voltage, can be prevented frombeing promoted and off-state current can be reduced.

Further, when an oxide semiconductor containing an impurity is subjectedto a gate bias-temperature stress test (BT test) for 12 hours underconditions that the temperature is 85° C. and the voltage applied to thegate is 2×10⁶ V/cm, a bond between the impurity and a main component ofthe oxide semiconductor is cut by a high electric field (B: bias) and ahigh temperature (T: temperature), and a generated dangling bond inducesdrift of threshold voltage (V_(th)). However, in the above-describedmanner, by improving the interfacial characteristics between the gateinsulating film and the oxide semiconductor film and removingimpurities, particularly hydrogen, water, and the like, in the oxidesemiconductor film as much as possible, a transistor which remainsstable even with respect to the BT test can be obtained.

Detailed description on the heat treatment apparatus used for the secondheat treatment is already made in Embodiment 1 and therefore omittedhere.

It is preferable that in the heat treatment, moisture, hydrogen, or thelike be not contained in nitrogen or a rare gas such as helium, neon, orargon. The purity of nitrogen or the rare gas such as helium, neon, orargon which is introduced into the heat treatment apparatus ispreferably set to be 6N (99.9999%) or higher, far preferably 7N(99.99999%) or higher (that is, the impurity concentration is preferably1 ppm or lower, far preferably 0.1 ppm or lower).

However, in the oxide semiconductor film 205, an oxygen defect isgenerated in addition to removal of water or hydrogen by the first heattreatment, and oxygen can be sufficiently supplied to theoxygen-deficient oxide semiconductor film by the addition of oxygen byan ion implantation method, an ion doping method, or the like. Further,since hydrogen or water removed by the first heat treatment is not acomponent element of the oxide semiconductor but a so-called impurityand oxygen added later is one component element of the oxidesemiconductor, a structure which satisfies the stoichiometriccomposition ratio can be obtained. Therefore, by performing the secondheat treatment after the first heat treatment and the addition ofoxygen, the damaged crystal region 206 can be repaired, and crystalgrowth is promoted from the superficial portion of the oxidesemiconductor film 208 to a larger depth of the oxide semiconductorfilm, so that a crystal region 209 which is expanded to a deeper portionof the oxide semiconductor film 208 can be formed. Further, crystalgrowth is further promoted by this second heat treatment as compared tothe first heat treatment, so that in the crystal region 209, crystalgrains are adjacent to each other and metal elements each of which is acomponent element of the oxide semiconductor are continuous, that is,continuously in contact with each other between crystal grains which areadjacent to each other. Accordingly, in a transistor in which a channelformation region is formed using the above-described crystal region, thepotential barrier at a crystal grain boundary is low, so that excellentcharacteristics such as high mobility and high withstand voltage can beachieved.

The oxide semiconductor film 208 shown in FIG. 5D includes an amorphousregion 210 which is mainly amorphous and the crystal region 209 formedin the superficial portion of the oxide semiconductor film 208.

Further, the crystal region 209 is more stable than the amorphous region210; therefore, when the crystal region 209 is included in thesuperficial portion of the oxide semiconductor film 208, the entry ofimpurities (e.g., hydrogen, water, a hydroxy group, hydride, or thelike) into the amorphous region 210 can be suppressed. Thus, thereliability of the oxide semiconductor film 208 can be improved.

Although the oxide semiconductor film 208 includes the crystal region209 and the amorphous region 210 in this embodiment, the oxidesemiconductor film 208 may be almost wholly occupied by the crystalregion 209. Even in the case where the oxide semiconductor film 208includes the crystal region 209 and the amorphous region 210, the depthto which the crystal region 209 reaches from the top surface of theoxide semiconductor film 208 is not limited that of the structure shownin FIGS. 5A to 5E and 6A to 6C.

Through the above-described process, the concentration of hydrogen inthe oxide semiconductor film can be reduced and the oxide semiconductorfilm can be highly purified. Thus, the oxide semiconductor film can bestabilized. In addition, heat treatment at a temperature which is lowerthan or equal to the glass transition temperature makes it possible toform an oxide semiconductor film with a wide band gap in which carrierdensity is extremely low. Therefore, a transistor can be manufacturedusing a large-sized substrate, so that mass productivity can beincreased. In addition, by using the oxide semiconductor film whosehydrogen concentration is reduced and which is highly purified, it ispossible to form a transistor with a high withstand voltage, lessshort-channel effect, and a high on/off ratio.

The amorphous region 210 is mainly an amorphous oxide semiconductorfilm. The word “mainly” means, for example, a state where the occupancyis 50% or more, and means a state where the amorphous region 210 isoccupied by the amorphous oxide semiconductor film at an occupancy of50% or more by volume (or weight) in this case. In other words, theamorphous region in some cases includes crystals of an oxidesemiconductor film other than an amorphous oxide semiconductor film, andthe occupancy thereof is preferably less than 50% by volume (or weight).However, the occupancy is not limited to the above.

In the case where the In—Ga—Zn—O-based oxide semiconductor film is usedas a material of the oxide semiconductor film, the composition of theabove-described amorphous region 210 is preferably set so that a Zncontent (atomic %) is larger than an In or Ga content (atomic %). Such acomposition makes it easy to form the crystal region 209 with apredetermined composition.

Next, as shown in FIG. 5E, a second electrode 211 is formed over theoxide semiconductor film 208. The similar mode of the first electrode202 can be applied to a material and the structure of a conductive filmused as the second electrode 211. A manufacturing method of the secondelectrode 211 can be carried out in a similar manner to that of thefirst electrode 202.

In this embodiment, a resist mask is formed over the conductive filmwhich serves as the second electrode 211 in a photolithography step, andthe conductive film is etched using the resist mask, so that the secondelectrode 211 is formed. Here, as the conductive film that serves as thesecond electrode 211, a 50-nm-thick titanium film, a 100-nm-thickaluminum film, and a 50-nm-thick titanium film are stacked in thisorder. The angle formed between the oxide semiconductor film 208 and anend portion of the second electrode 211 may be greater than or equal to30° and less than or equal to 60°, preferably greater than or equal to40° and less than or equal to 50°, which is preferable because thecoverage with a gate insulating film to be formed later can be improved.Then, the second electrode 211 is formed away from the first electrode202 so as not to be in contact with the first electrode 202.

One of the first electrode 202 and the second electrode 211 functions asa source electrode of a transistor and the other functions as a drainelectrode thereof.

Heat treatment may be performed after the formation of the secondelectrode 211. The temperature of the heat treatment is higher than orequal to 400° C. and lower than or equal to 850° C., preferably higherthan or equal to 400° C. and lower than a strain point of the substrate.In this embodiment, the substrate is introduced into an electric furnacethat is one of heat treatment apparatuses, and heat treatment isperformed on the oxide semiconductor film 208 in an inert gasatmosphere, such as a nitrogen atmosphere or a rare gas atmosphere, at450° C. for one hour, and then the oxide semiconductor film is notexposed to air so that hydrogen, water, a hydroxyl group, hydride, orthe like can be prevented from entering the oxide semiconductor film,whereby hydrogen concentration is further reduced, and the oxidesemiconductor film is highly purified, so that an i-type or asubstantially i-type oxide semiconductor film can be obtained.

It is preferable that in the heat treatment, hydrogen, water, a hydroxylgroup, a hydride, or the like be not contained in nitrogen or a rare gassuch as helium, neon, or argon. The purity of nitrogen or the rare gassuch as helium, neon, or argon which is introduced into the heattreatment apparatus is preferably set to be 6N (99.9999%) or higher, farpreferably 7N (99.99999%) or higher (that is, the impurity concentrationis preferably 1 ppm or lower, far preferably 0.1 ppm or lower).

FIG. 7A is a top view of the first electrode 202, the oxidesemiconductor film 208, and the second electrode 211 shown in FIG. 5E. Across-sectional view along dashed line B1-B2 in FIG. 7A corresponds toFIG. 5E.

Next, as shown in FIG. 6A, a gate insulating film 212 is formed to coverthe first electrode 202, the oxide semiconductor film 208, and thesecond electrode 211. A gate electrode 213 is formed over the gateinsulating film 212. The gate insulating film 212 can be formed to havea single-layer structure or a stacked-layer structure of one or moreselected from a silicon oxide film, a silicon nitride film, a siliconoxynitride film, a silicon nitride oxide film, an aluminum oxide film,an aluminum nitride film, an aluminum oxynitride film, an aluminumnitride oxide film, a hafnium oxide film, and a tantalum oxide film by aplasma CVD method, a sputtering method, or the like.

When the gate insulating film 212 is formed using a high-k material suchas hafnium silicate (HfSiO_(x)), HfSi_(x)O_(y) to which N is added,hafnium aluminate (HfAlO_(x)) to which N is added, hafnium oxide, oryttrium oxide, gate leakage can be reduced. Further, a stacked-layerstructure can be used in which a high-k material and one or moreselected from a silicon oxide film, a silicon nitride film, a siliconoxynitride film, a silicon nitride oxide film, and an aluminum oxidefilm are stacked. The thickness of the gate insulating film 212 may begreater than or equal to 50 nm and less than or equal to 500 nm. Leakagecurrent can be reduced by increasing the thickness of the gateinsulating film 212.

It is preferable that the gate insulating film 212 include impuritiessuch as moisture or hydrogen as little as possible. In the case where asilicon oxide film is formed by a sputtering method, a silicon target ora quartz target is used as a target and oxygen or a mixed gas of oxygenand argon is used as a sputtering gas.

An oxide semiconductor that is made to be i-type or substantially i-type(an oxide semiconductor that is highly purified) by removal ofimpurities is extremely sensitive to an interface state and an interfaceelectric charge; thus, an interface between the oxide semiconductor andthe gate insulating film 212 is important. Thus, a gate insulating film(GI) which is to be in contact with the highly purified oxidesemiconductor needs to have high quality.

For example, high-density plasma CVD using microwaves (2.45 GHz) ispreferable because a dense high-quality insulating film having highwithstand voltage can be formed. This is because an interface state canbe reduced and interface characteristics can be favorable when thehighly purified oxide semiconductor and the high quality gate insulatingfilm are in contact with each other.

Needless to say, other film formation methods, such as a sputteringmethod or a plasma CVD method, can be applied as long as a high-qualityinsulating film can be formed as the gate insulating film 212. The filmquality of the gate insulating film 212 and/or properties of aninterface with an oxide semiconductor thereof may be modified by heattreatment performed after film deposition. In any case, any insulatingfilm can be used as long as film quality as a gate insulating film ishigh, interface state density with an oxide semiconductor is decreased,and a favorable interface can be formed.

The gate insulating film 212 may have a structure in which an insulatingfilm formed using a material having a high barrier property and aninsulating film having lower proportion of nitrogen such as a siliconoxide film or a silicon oxynitride film are stacked. In that case, theinsulating film such as a silicon oxide film or a silicon oxynitridefilm is formed between the insulating film having a barrier property andthe oxide semiconductor film. As the insulating film having a highbarrier property, a silicon nitride film, a silicon nitride oxide film,an aluminum nitride film, an aluminum nitride oxide film, or the likecan be given, for example. With the insulating film having a barrierproperty, impurities in an atmosphere, such as moisture or hydrogen, orimpurities included in the substrate, such as an alkali metal or a heavymetal, can be prevented from entering the oxide semiconductor film, thegate insulating film 212, or the interface between the oxidesemiconductor film and another insulating film and the vicinity thereof.In addition, the insulating film having lower proportion of nitrogensuch as a silicon oxide film or a silicon oxynitride film may be formedso as to be in contact with the oxide semiconductor film, so that theinsulating film formed using a material having a high barrier propertycan be prevented from being in contact with the oxide semiconductor filmdirectly.

For example, a stacked-layer film with a thickness of 100 nm may beformed as the gate insulating film 212 as follows: a silicon oxide film(SiO_(X) (X>0)) with a thickness greater than or equal to 5 nm and lessthan or equal to 300 nm is formed as a first gate insulating film, and asilicon nitride film (SiN_(Y) (Y>0)) with a thickness greater than orequal to 50 nm and less than or equal to 200 nm is stacked by asputtering method over the first gate insulating film as a second gateinsulating film. In this embodiment, the silicon oxide film having athickness of 100 nm is formed by an RF sputtering method under thefollowing conditions: the pressure is 0.4 Pa; the high-frequency poweris 1.5 kW; and the atmosphere contains oxygen and argon (oxygen flowrate of 25 sccm:argon flow rate of 25 sccm=1:1).

Further, in order that hydrogen, a hydroxyl group, and moisture may becontained in the gate insulating film 212 as little as possible, it ispreferable that the substrate 200 provided for the first electrode 202,the oxide semiconductor film 208, and the second electrode 211 bepreheated in a preheating chamber of a sputtering apparatus aspretreatment for film formation so that impurities such as hydrogen andmoisture adsorbed to the substrate 200 are removed and exhausted. Thetemperature of the preheating is higher than or equal to 100° C. andlower than or equal to 400° C., preferably higher than or equal to 150°C. and lower than or equal to 300° C. As an exhaustion unit provided inthe preheating chamber, a cryopump is preferable. This preheatingtreatment can be omitted.

After the gate insulating film 212 is formed, heat treatment may beperformed. The heat treatment is performed in an inert gas atmosphere(nitrogen, helium, neon, argon, or the like) at a temperature,preferably, higher than or equal to 200° C. and lower than or equal to400° C., for example, at a temperature higher than or equal to 250° C.and lower than or equal to 350° C. In this embodiment, heat treatmentfor 1 hour at 250° C. in a nitrogen atmosphere is performed. By the heattreatment performed on the state where silicon oxide contained in thegate insulating film 212 is in contact with the oxide semiconductor film208, oxygen is supplied from the silicon oxide even when an oxygendefect is generated in the second heat treatment, whereby the number ofoxygen defects which form donors can be reduced, a structure whichsatisfies the stoichiometric composition ratio can be obtained, and theoxide semiconductor film 208 is made to be an i-type or substantiallyi-type. The timing of this heat treatment is not particularly limited aslong as it is after the formation of the gate insulating film 212, andmay be performed after another step, for example, after any one of thegate electrode 213, an insulating film 214, and wirings 215, 216, and217 is formed. This heat treatment can be performed without increasingthe number of manufacturing steps by doubling as another step such as aheat treatment for reduction of the resistance of a transparentconductive film.

The gate electrode 213 can be formed to have a single-layer structure ora stacked-layer structure using one or more conductive films using ametal material such as molybdenum, titanium, chromium, tantalum,tungsten, neodymium, or scandium or an alloy material which contains anyof these metal materials as its main component, or a nitride whichcontains any of these metals. Aluminum or copper can be used as one ofthe metal material if aluminum or copper can withstand a temperature ofheat treatment performed in a later process. Aluminum or copper ispreferably combined with a refractory metal material so as to prevent aheat resistance problem and a corrosive problem. As the refractory metalmaterial, molybdenum, titanium, chromium, tantalum, tungsten, neodymium,scandium, or the like can be used.

For example, as a two-layer structure of the gate electrode 213, thefollowing structure is preferable: a two-layer structure in which amolybdenum film is stacked over an aluminum film, a two-layer structurein which a molybdenum film is stacked over a copper film, a two-layerstructure in which a titanium nitride film or a tantalum nitride film isstacked over a copper film, or a two-layer structure in which a titaniumnitride film and a molybdenum film are stacked. As a three-layerstructure of the gate electrode 213, the following structure ispreferable: a stacked structure in which an aluminum film, an alloy filmof aluminum and silicon, an alloy film of aluminum and titanium, or analloy film of aluminum and neodymium is sandwiched by any two filmsselected from a tungsten film, a tungsten nitride film, a titaniumnitride film, and a titanium film.

Further, a light-transmitting oxide conductive film of indium oxide, analloy of indium oxide and tin oxide, an alloy of indium oxide and zincoxide, zinc oxide, aluminum zinc oxide, aluminum zinc oxynitride,gallium zinc oxide, or the like may be used for the gate electrode 213,so that the aperture ratio of a pixel portion can be increased.

The gate electrode 213 is formed to a thickness of 10 nm to 400 nm,preferably 100 nm to 200 nm. In this embodiment, a conductive film forthe gate electrode is formed to a thickness of 150 nm by a sputteringmethod using a tungsten target, and then, the conductive film isprocessed (patterned) into an appropriate shape by etching; in such amanner, the gate electrode 213 is formed. The gate electrode 213 isformed at least so as to overlap with the end portion of the oxidesemiconductor film 208 with the gate insulating film 212 providedtherebetween. In the end portion of the oxide semiconductor film 208, achannel formation region is formed in a portion 218 which overlaps withthe gate electrode 213 with the gate insulating film 212 providedtherebetween. It is preferable that an end portion of the gate electrode213 be tapered because coverage with the insulating film 214 formedthereover is improved. Note that a resist mask may be formed by aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing costs can be reduced.

Next, as shown in FIG. 6B, the insulating film 214 is formed to coverthe first electrode 202, the oxide semiconductor film 208, the secondelectrode 211, the gate insulating film 212, and the gate electrode 213,and then, contact holes 221, 222, and 223 are formed. The insulatingfilm 214 preferably includes no impurities such as moisture or hydrogenas much as possible, and may be formed using a single-layer insulatingfilm or a plurality of insulating films stacked. The insulating film 214is formed, for example, using an oxide insulating film such as a siliconoxide film, a silicon oxynitride film, an aluminum oxide film, or analuminum oxynitride film; or a nitride insulating film such as a siliconnitride film, a silicon nitride oxide film, an aluminum nitride film, oran aluminum nitride oxide film.

Alternatively, an oxide insulating film and a nitride insulating filmcan be stacked. An insulating film having a high barrier property, forexample, a silicon nitride film, a silicon nitride oxide film, analuminum nitride film, or an aluminum nitride oxide film may be used forthe insulating film 214, so that impurities such as moisture or hydrogencan be prevented from entering the oxide semiconductor film 208, thegate insulating film 212, or the interface between the oxidesemiconductor film 208 and another insulating film and the vicinitythereof.

In this embodiment, the insulating film 214 has a structure in which asilicon nitride film having a thickness of 100 nm formed by a sputteringmethod is stacked over a silicon oxide film having a thickness of 200 nmformed by a sputtering method. Note that when the insulating film 214 isformed by a sputtering method, the substrate 200 may be heated to atemperature of 100° C. to 400° C., a high-purity sputtering gas whichcontains nitrogen, from which hydrogen, water, a hydroxyl group,hydride, or the like is removed may be introduced, and an insulatingfilm may be formed using a silicon target. Also in that case, theinsulating film is preferably formed while hydrogen, water, a hydroxylgroup, hydride, or the like remaining in the treatment chamber isremoved.

After the insulating film 214 is formed, heat treatment may beperformed. The heat treatment is performed in an inert gas atmosphere(nitrogen, helium, neon, argon, or the like) at a temperature,preferably, higher than or equal to 200° C. and lower than or equal to400° C., for example, at a temperature higher than or equal to 250° C.and lower than or equal to 350° C.

The contact holes 221, 222, and 223 are formed as follows: a resist maskis formed by a photolithography step and parts of the gate insulatingfilm 212 and the insulating film 214 are selectively etched using theresist mask. A part of the gate electrode 213 is exposed in the contacthole 221; a part of the second electrode 211 is exposed in the contacthole 222; and a part of the second electrode 213 is exposed in thecontact hole 222. At the time of the formation of these contact holes, acontact hole so as to expose the first electrode 202 may be formed in aregion of the first electrode 202, which is not covered with the gateelectrode 213.

Next, as shown in FIG. 6C, a conductive film is formed over theinsulating film 214 to cover the contact holes 221, 222, and 223 and isprocessed into a desired shape by etching or the like, so that thewirings 215, 216, and 217 are formed. A resist mask used for the etchingmay be formed by an inkjet method. No photomask is used when a resistmask is formed by an ink-jet method; therefore, manufacturing costs canbe reduced.

The wiring 215 is connected to the gate electrode 213 through thecontact hole 221. The wiring 216 is connected to the second electrode211 through the contact hole 222. The wiring 217 is connected to thegate electrode 213 through the contact hole 223. At the time of theformation of these wirings, a wiring which is connected to the firstelectrode 202 through a contact hole may be formed.

The wirings 215, 216, and 217 can be formed using a conductive filmhaving a similar structure and a similar material to the first electrode202 by a similar manufacturing method to the first electrode 202.

Through the above-described process, a transistor 220 is formed.

FIG. 7B is a top view of the transistor 220 shown in FIG. 6C. Across-sectional view along dashed line B1-B2 in FIG. 7B corresponds toFIG. 6C. In FIG. 7B, a wiring 230 is a wiring formed at the same time asthe wirings 215, 216, and 217 and is connected to the first electrode202 through a contact hole 231.

In this manner, the concentration of hydrogen in the oxide semiconductorfilm can be reduced and the oxide semiconductor film can be highlypurified. Thus, the oxide semiconductor film can be stabilized. Inaddition, heat treatment at a temperature which is lower than or equalto the glass transition temperature makes it possible to form an oxidesemiconductor film with a wide band gap in which carrier density isextremely low. Therefore, a transistor can be manufactured using alarge-sized substrate, so that mass productivity can be increased. Inaddition, by using the oxide semiconductor film whose hydrogenconcentration is reduced and which is highly purified, it is possible toform a transistor with a high withstand voltage, less short-channeleffect, and a high on/off ratio.

In this embodiment, at least the part of the oxide semiconductor film208, which is formed in a region which is different from the secondelectrode 211 may be covered with the gate electrode 213. Further, theelectrode of the first electrode 202 or the second electrode 211, whichfunctions as a drain electrode, may be connected to the gate electrode213. When the electrode which functions as a drain electrode isconnected to the gate electrode 213, the transistor can function as adiode.

The terms of the “source electrode” and the “drain electrode” includedin the transistor interchange with each other depending on the polarityof the transistor or difference between the levels of potentials appliedto the respective electrodes. In general, in an n-channel transistor, anelectrode to which a lower potential is applied is called a sourceelectrode, and an electrode to which a higher potential is applied iscalled a drain electrode. Further, in a p-channel transistor, anelectrode to which a lower potential is applied is called a drainelectrode, and an electrode to which a higher potential is applied iscalled a source electrode. In this specification and the like, forconvenience, connection relation of the transistor is described assumingthat the source electrode and the drain electrode are fixed; however,actually, the terms of the source electrode and the drain electrodeinterchange with each other depending on relation between the abovepotentials.

Note that “connection” in this specification and the like refers toelectrical connection and corresponds to the state in which current orvoltage can be conducted.

Here, a drain withstand voltage of the transistor described in thisembodiment is described below.

When the electric field in the semiconductor reaches a certain thresholdvalue, impact ionization occurs, and carriers accelerated by the highelectric field impact crystal lattices in a depletion layer, therebygenerating pairs of electrons and holes. As the electric field becomeshigher, the pairs of electrons and holes generated by the impactionization are further accelerated by the electric field, and the impactionization is repeated, which causes an avalanche breakdown in whichcurrent is increased exponentially. The impact ionization occurs becausecarriers (electrons and holes) have kinetic energy that is larger thanor equal to the band gap of the semiconductor. Therefore, the electricfield which causes the impact ionization is increased as the band gap isincreased.

Since the band gap of an oxide semiconductor is 3.15 eV, which is largerthan the band gap of silicon, i.e., 1.74 eV, the avalanche breakdown isunlikely to occur. Therefore, a transistor using the oxide semiconductorhas a high drain withstand voltage, and an exponential sudden increaseof on-state current is unlikely to occur even when a high electric fieldis applied.

Next, hot-carrier degradation of a transistor using an oxidesemiconductor is described.

The hot-carrier degradation means deterioration of transistorcharacteristics, e.g., variation in the threshold voltage or a gateleakage current, which is caused as follows: electrons that areaccelerated to be rapid are injected into a gate insulating film in thevicinity of a drain in a channel and become fixed electric charge orform trap levels at the interface between the gate insulating film andthe oxide semiconductor. The factors of the hot-carrier degradation are,for example, channel-hot-electron injection (CHE injection) anddrain-avalanche-hot-carrier injection (DAHC injection).

Since the band gap of silicon is narrow, electrons are likely to begenerated like an avalanche owing to an avalanche breakdown, and thenumber of electrons that are accelerated to be so rapid as to go over apotential barrier to the gate insulating film is increased. However, theoxide semiconductor described in this embodiment has a wide band gap;therefore, the avalanche breakdown is unlikely to occur and resistanceto the hot-carrier degradation is higher than that of silicon. Note thatalthough the band gap of silicon carbide which is one of materialshaving high withstand voltage and that of an oxide semiconductor aresubstantially equal to each other, electrons are less likely to beaccelerated, hot-carrier degradation is less likely to be caused than inthe case of silicon carbide, and drain withstand voltage is high in theoxide semiconductor because the mobility of the oxide semiconductor istwo orders of magnitude smaller than that of silicon carbide.

From the above, the transistor using an oxide semiconductor has highdrain withstand voltage; specifically, such a transistor can have adrain withstand voltage greater than or equal to 100 V, preferablygreater than or equal to 500 V, far preferably greater than or equal to1 kV.

Comparison between a transistor using silicon carbide, which is atypical example of a transistor, and a transistor using an oxidesemiconductor is described below. Here, 4H—SiC is used as the siliconcarbide.

An oxide semiconductor and 4H—SiC are common in some points. One examplethereof is intrinsic carrier density. According to the Fermi-Diracdistribution, the intrinsic carrier density of the oxide semiconductoris estimated to about 10⁻⁷ cm⁻³, which is extremely low like the carrierdensity of 4H—SiC, i.e., 6.7×10⁻¹¹ cm⁻³.

In addition, the energy band gap of the oxide semiconductor is 3.0 eV to3.5 eV and that of 4H—SiC is 3.26 eV, which means that the oxidesemiconductor and the silicon carbide are both wide-gap semiconductors.

However, the manufacturing temperatures of a transistor using an oxidesemiconductor and a transistor using silicon carbide are largelydifferent from each other. Heat treatment at 1500° C. to 2000° C. isgenerally needed in the case of using silicon carbide. In contrast, inthe case of using an oxide semiconductor, manufacturing can be performedby heat treatment at 300° C. to 500° C. (lower than or equal to theglass transition temperature, about 700° C. at maximum), which allowsmanufacturing of a transistor over a large-sized substrate. In addition,throughput can be increased.

Further, a manufacturing process of the transistor using siliconcarbide, which uses a PN junction, involves a step of doping with animpurity that can be a donor or an acceptor (e.g., phosphorus or boron);therefore, the number of manufacturing steps is increased. On the otherhand, the transistor using an oxide semiconductor is not needed to beprovided with a PN junction; therefore, the number of manufacturingsteps can be decreased and the throughput can be improved, and further,a large-sized substrate can be used.

Note that considerable researches have been done on properties of oxidesemiconductors such as density of state (DOS) in the band gap; however,the research does not include the idea of sufficiently reducing the DOSitself. In this embodiment, a highly purified oxide semiconductor ismanufactured by removing water or hydrogen which might induce the DOSfrom the oxide semiconductor. This idea is based on the idea that theDOS itself is reduced sufficiently; thus, excellent industrial productscan be manufactured.

Further, it is also possible to form a more highly purified (i-type)oxide semiconductor by supplying oxygen to a dangling bond of metalwhich is generated by an oxygen defect and reducing the DOS due to theoxygen defect. For example, an oxide film containing an excessive amountof oxygen is formed in close contact with a channel formation region andoxygen is supplied from the oxide film, whereby the DOS due to an oxygendefect can be reduced.

It is said that a defect of the oxide semiconductor is caused by ashallow level of 0.1 eV to 0.2 eV below the conduction band due to anexcessive amount of hydrogen, a deep level due to lack of oxygen, or thelike. The technical idea that hydrogen is reduced as much as possibleand oxygen is sufficiently supplied in order to eliminate such a defectwould be right.

An oxide semiconductor is generally considered as an n-typesemiconductor; however, in this embodiment, an i-type oxidesemiconductor is realized by removing an impurity, particularly water orhydrogen. In this point, different from the case of silicon which ismade to be an i-type silicon by adding an impurity, one embodiment ofthe present invention includes a novel technical idea.

By making the oxide semiconductor an i-type semiconductor, favorabletemperature characteristics of the transistor can be obtained;specifically, in terms of the current vs. voltage characteristics of thetransistor, on-state current, off-state current, field-effect mobility,an S value, and a threshold voltage are hardly fluctuated attemperatures ranging from −25° C. to 150° C. That is, the current vs.voltage characteristics are hardly degraded by the temperature.

In the transistor using an oxide semiconductor which is described inthis embodiment, mobility is about two orders of magnitude smaller thanthat of a transistor using silicon carbide; however, a current value anddevice characteristics of the transistor can be improved by increasingthe drain voltage and the channel width (W).

A technical idea of this embodiment is that an impurity is not added toan oxide semiconductor and the oxide semiconductor itself is highlypurified by removing an impurity such as water or hydrogen which existstherein. In other words, the oxide semiconductor is highly purified byremoving water or hydrogen which forms a donor level, reducing oxygendeficiency, and sufficiently supplying oxygen that is a main componentof the oxide semiconductor.

In an oxide semiconductor as deposition, a hydrogen concentration on theorder of 10²⁰ cm⁻³ is measured by secondary ion mass spectrometry(SIMS). In accordance with the present invention is to highly purify anoxide semiconductor to obtain an electrically i-type (intrinsic) oxidesemiconductor by removing an impurity such as water or hydrogen whichforms a donor level and further by adding oxygen (one of components ofthe oxide semiconductor) which decreases at the same time as the removalof water or hydrogen, to the oxide semiconductor.

In this embodiment, it is preferable that the amount of water andhydrogen in the oxide semiconductor and the number of carriers in theoxide semiconductor be small as much as possible. In other words, thecarrier density is preferably less than 1×10¹⁴ cm⁻³, far preferably lessthan 1×10¹² cm⁻³, still far preferably less than 1×10¹¹ cm⁻³ that isless than the measurement limit. Further, in the technical idea of thisembodiment, an ideal carrier density is 0 or as close to 0 as possible.By reducing the number of and preferably removing carriers in the oxidesemiconductor, the oxide semiconductor is made to function as a paththrough which carriers pass in the transistor. As a result, the oxidesemiconductor is made to be an i-type (intrinsic) oxide semiconductorwhich is highly purified and includes an extremely small number of or nocarriers, whereby the off-state current can be extremely small when thetransistor is off, which is the technical idea of this embodiment.

In addition, when the oxide semiconductor functions as a path and theoxide semiconductor itself is an i-type (intrinsic) oxide semiconductorwhich is highly purified so as to include an extremely small number ofor no carriers, carriers are supplied from a source and drainelectrodes.

The transistor having the structure described in Embodiment 2 can lessoccupy a substrate surface than a horizontal transistor in which achannel is formed substantially in parallel with a substrate asdescribed in Embodiment 1. As a result of this, it is possible tominiaturize the transistor.

As described above, the oxide semiconductor film is highly purified sothat an impurity that is not a main component of the oxide semiconductorfilm, typically hydrogen, water, a hydroxy group, or hydride, iscontained as little as possible, whereby good operation of thetransistor can be obtained. In particular, withstand voltage can behigher, a short channel effect can be reduced, and a high on-off ratiocan be realized.

Further, similarly to the crystal region 109 in Embodiment 1, c-axes ofcrystals in the crystal region 209 in the superficial portion areoriented in a direction which is substantially perpendicular to the topsurface of the oxide semiconductor film 208 and the crystals areadjacent to each other. Therefore, as described in Embodiment 1, owingto the crystal region 209, the electrical characteristics of the oxidesemiconductor film 208 in a direction which is parallel to the topsurface thereof are improved. According to one embodiment of the presentinvention, in the crystal region, crystal grains are adjacent to eachother and metal elements each of which is a component element of theoxide semiconductor are continuous, that is, continuously in contactwith each other between crystal grains which are adjacent to each other.Therefore, electrical characteristics in a direction parallel to the topsurface of the oxide semiconductor film 208 can be further improved.Therefore, the carrier mobility in the superficial portion of the oxidesemiconductor film 208 is improved, so that the field-effect mobility ofthe transistor using the oxide semiconductor film 208 is increased andgood electrical characteristics can be achieved.

The crystal structure of the crystal region 209 is not limited to theabove-described structure, and the crystal region 209 may include acrystal having another structure. For example, in the case of using anIn—Ga—Zn—O-based oxide semiconductor material, crystals of In₂Ga₂ZnO₇,InGaZn₅O₈, or the like may be included in addition to crystals ofInGaZnO₄ crystals. Needless to say, the case where the crystals ofInGaZnO₄ exist in the whole crystal region 209 is more effective andmore preferable.

Further, the crystal region 209 is more stable than the amorphous region210; therefore, when the crystal region 209 is included in thesuperficial portion of the oxide semiconductor film 208, the entry ofimpurities (e.g., hydrogen, water, a hydroxy group, hydride, or thelike) into the amorphous region 210 can be suppressed. Thus, thereliability of the oxide semiconductor film 208 can be improved.

This embodiment can be implemented by being combined as appropriate withthe above-described embodiment.

Embodiment 3

In Embodiment 3, a structure and a method for manufacturing asemiconductor device will be described using a bottom-gate transistorhaving a channel protective structure as an example. Embodiment 1 can beapplied to the same portions as Embodiment 1 or portions havingfunctions similar to those of Embodiment 1, and therefore, descriptionthereof is not omitted.

The state shown in FIG. 1E described in Embodiment 1 is obtained throughthe process up to and including the second heat treatment in a similarmanner. Next, as shown in FIG. 8A, a channel protective film 130 isformed over the oxide semiconductor film 108 so as to overlap with aregion of the oxide semiconductor film 108, which overlaps with the gateelectrode 101, that is, so as to overlap with a channel formationregion. The channel protective film 130 can prevent the portion of theoxide semiconductor film 108, which serves as the channel formationregion, from being damaged in a later step (for example, reduction inthickness due to plasma or an etchant in etching). Accordingly,reliability of the transistor can be improved.

The channel protective film 130 can be formed using an inorganicmaterial including oxygen (such as silicon oxide, silicon oxynitride, orsilicon nitride oxide). The channel protective film 130 can be formed bya vapor deposition method such as a plasma enhanced CVD method or athermal CVD method, or a sputtering method. After the deposition of thechannel protective film 130, the shape thereof is processed by etching.In this embodiment, the channel protective film 130 is formed in such amanner that a silicon oxide film is formed by a sputtering method andprocessed by etching using a mask formed by photolithography.

After the channel protective film 130 is formed, heat treatment may beperformed. The heat treatment is performed in an inert gas atmosphere(nitrogen, helium, neon, argon, or the like) at a temperature,preferably, higher than or equal to 200° C. and lower than or equal to400° C., for example, at a temperature higher than or equal to 250° C.and lower than or equal to 350° C. In this embodiment, heat treatmentfor 1 hour at 250° C. in a nitrogen atmosphere is performed. By the heattreatment performed on the state where the portion of the oxidesemiconductor film 108 which forms the channel formation region is incontact with the channel protective film 130 that is an insulating filmcontaining oxygen, oxygen is supplied to the oxide semiconductor film108, whereby the region of the oxide semiconductor film 108, which is incontact with the channel protective film 130 can be selectively made anoxygen-excess state. Consequently, even when an oxygen defect occurs dueto the second heat treatment in a region of the oxide semiconductor film108, which is in contact with at least the channel protective film 130,the number of oxygen defects which become donors is reduced, a structurewhich satisfies the stoichiometric composition ratio can be obtained,and a channel formation region which overlaps with the gate electrode101 becomes i-type or substantially i-type, which leads to improvementof the electrical characteristics of the transistor and suppression ofvariation of the electrical characteristics. The timing of this heattreatment is not particularly limited as long as it is after theformation of the channel protective film 130, and can be performedwithout increasing the number of manufacturing steps by doubling asanother step such as a heat treatment for a formation of a resin film ora heat treatment for reduction of the resistance of a transparentconductive film.

Next, as shown in FIG. 8B, a conductive film which forms a sourceelectrode and a drain electrode (including a wiring formed from the samelayer as the source electrode or the drain electrode) is formed over theoxide semiconductor film 108 and is processed into an appropriate shapeby etching or the like to form a source electrode 131 and a drainelectrode 132. The description on the source electrode 111 and the drainelectrode 112 described in Embodiment 1 can be referred to for amaterial, the structure, and the thickness of each of the sourceelectrode 131 and the drain electrode 132.

The source electrode 131 and the drain electrode 132 are in contact withthe crystal region 109 of the oxide semiconductor film 108. Owing tothis contact between the highly conductive crystal region 109 and eachof the source electrode 131 and the drain electrode 132, the contactresistance between each of the source electrode 131 and the drainelectrode 132 and the oxide semiconductor film 108 can be reduced, sothat the on-state current of the transistor formed can be increased.

Next, plasma treatment is performed thereon, using a gas such as N₂O,N₂, or Ar. By the plasma treatment, adsorbed water or the like whichattaches to an exposed surface of the oxide semiconductor film isremoved. Plasma treatment may be performed using a mixture gas of oxygenand argon as well.

After the plasma treatment, as shown in FIG. 8C, an insulating film 133is formed to cover the source electrode 131, the drain electrode 132,the channel protective film 130, and the oxide semiconductor film 108.The description on the insulating film 113 described in Embodiment 1 canbe referred to for a material, the thickness, the structure, and themanufacturing method of the insulating film 133.

After the insulating film 133 is formed, heat treatment may beperformed. The heat treatment is performed in an inert gas atmosphere(nitrogen, helium, neon, argon, or the like) at a temperature,preferably, higher than or equal to 200° C. and lower than or equal to400° C., for example, at a temperature higher than or equal to 250° C.and lower than or equal to 350° C. In this embodiment, heat treatmentfor 1 hour at 250° C. in a nitrogen atmosphere is performed.

Through the above-described process, a transistor 140 is formed.

Although the oxide semiconductor film 108 includes the crystal region109 and the amorphous region 110 in this embodiment, the oxidesemiconductor film 108 may be almost wholly occupied by the crystalregion 109. Even in the case where the oxide semiconductor film 108includes the crystal region 109 and the amorphous region 110, the depthto which the crystal region 109 reaches from the top surface of theoxide semiconductor film 108 is not limited that of the structure shownin FIGS. 8A to 8C.

FIG. 9 is a top view of the transistor 140 shown in FIG. 8C. Across-sectional view along dashed line C1-C2 in FIG. 9 corresponds toFIG. 8C.

The transistor 140 formed according to the above-described manufacturingmethod includes the gate electrode 101, the gate insulating film 102over the gate electrode 101, the oxide semiconductor film 108 over thegate insulating film 102, the channel protective film 130 over the oxidesemiconductor film 108, and the source electrode 131 and the drainelectrode 132 over the oxide semiconductor film 108. The transistor 140may further include the insulating film 133 over the oxide semiconductorfilm 108, the source electrode 131 and the drain electrode 132, and thechannel protective film 130.

Although the transistor 140 is described as a single-gate transistor, amulti-gate transistor including a plurality of channel formation regionscan be formed when needed.

Next, a conductive film may be formed over the insulating film 133 andmay be patterned so that a back gate electrode 145 is formed so as tooverlap with the oxide semiconductor film 108 as shown in FIG. 10A. Theback gate electrode 145 can be formed using a material and a structurewhich are similar to those of the gate electrode 101 or the sourceelectrode 131 or the drain electrode 132.

The thickness of the back gate electrode 145 is set to 10 nm to 400 nm,preferably 100 nm to 200 nm. In this embodiment, a conductive film inwhich a titanium film, an aluminum film, and a titanium film are stackedis formed. Then, a resist mask is formed by a photolithography method,and an unnecessary portion is removed by etching so that the conductivefilm is processed (patterned) into an appropriate shape; thus, the backgate electrode 145 is formed.

Next, as shown in FIG. 10B, an insulating film 146 is formed so as tocover the back gate electrode 145. The insulating film 146 is preferablyformed using a material with a high barrier property that can preventmoisture, hydrogen, oxygen, and the like in an atmosphere from affectingthe characteristics of the transistor 140. For example, the insulatingfilm having a high barrier property can be formed to have a single-layerstructure or a stacked-layer structure of a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, or the like by a plasma CVD method, a sputteringmethod, or the like. In order to obtain an effect of a barrier property,the insulating film 146 is preferably formed to a thickness of 15 nm to400 nm, for example.

In this embodiment, an insulating film with a thickness of 300 nm isformed by a plasma CVD method. The insulating film is formed under thefollowing conditions: the flow rate of a silane gas is 4 sccm; the flowrate of nitrous oxide is 800 sccm; and the substrate temperature is 400°C.

A top view of the semiconductor device shown in FIG. 10B is FIG. 10C.FIG. 10B is a cross-sectional view along dashed line C1-C2 in FIG. 10C.

Although the back gate electrode 145 covers the oxide semiconductor film108 entirely in FIG. 10B, one embodiment of the present invention is notlimited to this structure. The back gate electrode 145 overlaps with atleast part of the channel formation region included in the oxidesemiconductor film 108.

The back gate electrode 145 may be electrically insulated to be in afloating state, or may be in a state where the back gate electrode 145is supplied with a potential. In the latter case, to the back gateelectrode 145, a potential which is the same level as the gate electrode101 may be applied, or a fixed potential such as ground may be applied.The level of the potential supplied to the back gate electrode 145 iscontrolled, whereby the threshold voltage of the transistor 140 can becontrolled.

This embodiment can be implemented by being combined as appropriate withthe above-described embodiments.

Embodiment 4

In Embodiment 4, a structure of a semiconductor display device which isreferred to as electronic paper or digital paper that is one of thesemiconductor display devices formed using a manufacturing method of thepresent invention will be described.

A display element which can control grayscale by voltage application andhas a memory property is used for electronic paper. Specifically, in thedisplay element used for electronic paper, the following display elementcan be used: a non-aqueous electrophoretic display element; a displayelement using a PDLC (polymer dispersed liquid crystal) method, in whichliquid crystal droplets are dispersed in a high polymer material betweentwo electrodes; a display element which includes chiral nematic liquidcrystal or cholesteric liquid crystal between two electrodes; a displayelement which includes charged fine particles between two electrodes andemploys a particle-moving method by which the charged fine particles aremoved through fine particles by using an electric field; or the like.Non-aqueous electrophoretic display elements include in its category adisplay element in which a dispersion liquid, in which charged fineparticles are dispersed, is sandwiched between two electrodes; a displayelement in which a dispersion liquid in which charged fine particles aredispersed is included over two electrodes between which an insulatingfilm is interposed; a display element in which twisting balls havinghemispheres which are different colors which charge differently aredispersed in a solvent between two electrodes; a display element whichincludes microcapsules, in which a plurality of charged fine particlesare dispersed in a solution, between two electrodes; and the like.

FIG. 13A is a top view of a pixel portion 700, a signal line drivercircuit 701, and a scan line driver circuit 702 of electronic paper.

The pixel portion 700 includes a plurality of pixels 703. A plurality ofsignal lines 707 is led into the pixel portion 700 from the signal linedriver circuit 701. A plurality of scan lines 708 is led into the pixelportion 700 from the scan line driver circuit 702.

Each pixel 703 includes a transistor 704, a display element 705, and astorage capacitor 706. A gate electrode of the transistor 704 isconnected to one of the scan lines 708. One of a source electrode and adrain electrode of the transistor 704 is connected to one of the signallines 707, and the other of the source electrode and the drain electrodeof the transistor 704 is connected to a pixel electrode of the displayelement 705.

In FIG. 13A, the storage capacitor 706 is connected in parallel to thedisplay element 705 so that voltage applied between the pixel electrodeand a counter electrode of the display element 705 may be stored;however, in the case where the memory property of the display element705 is high enough to maintain display, the storage capacitor 706 is notnecessarily provided.

Although a structure of an active matrix pixel portion in which onetransistor which serves as a switching element is provided in each pixelis illustrated in FIG. 13A, electronic paper of one embodiment of thepresent invention is not limited to this structure. A plurality oftransistors may be provided in each pixel.

In addition to the transistor, another element such as a capacitor, aresistor, or a coil may be provided.

A cross-sectional view of the display element 705 provided in each pixel703 is illustrated in FIG. 13B taking electrophoretic electronic paperhaving microcapsules as an example.

The display element 705 includes a pixel electrode 710, a counterelectrode 711, and a microcapsule 712 to which voltage is applied by thepixel electrode 710 and the counter electrode 711. A source electrode ora drain electrode 713 of the transistor 704 is connected to the pixelelectrode 710.

In the microcapsule 712, a positively charged white pigment such astitanium oxide and a negatively charged black pigment such as carbonblack are encapsulated together with a dispersion medium such as oil.Voltage is applied between the pixel electrode and the counter electrodein accordance with the voltage of a video signal applied to the pixelelectrode 710, and the black pigment and the white pigment are drawn toa positive electrode side and a negative electrode side, respectively,whereby grayscales can be displayed.

In FIG. 13B, the microcapsule 712 is fixed by a light-transmitting resin714 between the pixel electrode 710 and the counter electrode 711.However, one embodiment of the present invention is not limited to thisstructure, and a space formed by the microcapsule 712, the pixelelectrode 710, and the counter electrode 711 may be filled with a gassuch as air, an inert gas, or the like. In that case, the microcapsule712 is preferably fixed to one of or both the pixel electrode 710 andthe counter electrode 711 by an adhesive agent or the like.

The number of the microcapsules 712 included in the display element 705is not necessarily plural as illustrated in FIG. 13B. One displayelement 705 may have a plurality of the microcapsules 712, or aplurality of the display elements 705 may have one microcapsule 712. Forexample, two display elements 705 share one microcapsule 712, andpositive voltage and negative voltage are applied to the pixel electrode710 included in one of the display elements 705 and the pixel electrode710 included in the other display element 705, respectively. In thatcase, in the microcapsule 712 in a region overlapping with the pixelelectrode 710 to which positive voltage is applied, the black pigment isdrawn to the pixel electrode 710 side and the white pigment is drawn tothe counter electrode 711 side. On the other hand, in the microcapsule712 in a region overlapping with the pixel electrode 710 to whichnegative voltage is applied, the white pigment is drawn to the pixelelectrode 710 side and the black pigment is drawn to the counterelectrode 711 side.

Next, a specific driving method of electronic paper is described usingthe above-described electronic paper of the electrophoretic system as anexample.

The operation of the electronic paper can be described in accordancewith the following periods: an initialization period, a writing period,and a holding period.

First, the grayscale levels of each of the pixels in a pixel portion areset to be equal in the initialization period before a display image isswitched in order to initialize display elements. Initialization of thedisplay elements prevents a residual image from remaining. Specifically,in an electrophoretic system, displayed grayscale level is adjusted bythe microcapsule 712 included in the display element 705 such that thedisplay of each pixel is white or black.

In this embodiment, an operation for initialization in the case where aninitialization video signal for displaying black is input to a pixel andthen an initialization video signal for displaying white is input to thepixel is described. For example, in the case where image display isperformed with respect to the counter electrode 711 side in theelectronic paper of an electrophoretic system, voltage is applied to thedisplay element 705 such that the black pigment in the microcapsule 712moves to the counter electrode 711 side and the white pigment in themicrocapsule 712 moves to the pixel electrode 710 side. Then, voltage isapplied to the display element 705 such that the white pigment in themicrocapsule 712 moves to the counter electrode 711 side and the blackpigment in the microcapsule 712 moves to the pixel electrode 710 side.

Further, in the case where the initialization video signal is input tothe pixel only once, the white pigment and the black pigment in themicrocapsule 712 do not finish moving completely depending on thegrayscale level displayed before the initialization period; thus, it ispossible that a difference between grayscale levels for display ofpixels occurs even after the initialization period ends. Therefore, itis preferable that negative voltage −Vp with respect to common voltageVcom be applied to the pixel electrode 710 a plurality of times so thatblack is displayed and positive voltage Vp with respect to the commonvoltage Vcom be applied to the pixel electrode 710 a plurality of timesso that white is displayed.

In the case where grayscale levels for display before the initializationperiod differ depending on display elements of each of the pixels, theminimum number of times for inputting an initialization video signalalso varies. Accordingly, the number of times for inputting aninitialization video signal may be changed between pixels in accordancewith a grayscale level for display before the initialization period. Inthat case, the common voltage Vcom is preferably input to a pixel towhich the initialization video signal is not necessarily input.

In order for the voltage Vp or the voltage −Vp which is aninitialization video signal to be applied to the pixel electrode 710plural times, the following operation sequence is performed pluraltimes: the initialization video signal is input to a pixel of a lineincluding the scan line in a period during which a pulse of a selectionsignal is supplied to each scan line. The voltage Vp or the voltage −Vpwhich is an initialization video signal is applied to the pixelelectrode 710 plural times, whereby movement of the white pigment andthe black pigment in the microcapsule 712 converges in order to preventa difference of grayscale levels between pixels from occurring. Thus,initialization of a pixel in the pixel portion can be performed.

Note that in each pixel in the initialization period, black may bedisplayed after white is displayed instead of that white is displayedafter black is displayed. Alternatively, in each pixel in theinitialization period, black may be displayed after white is displayed;and further, after that white may be displayed.

Further, as for all of the pixels in the pixel portion, timing ofstarting the initialization period is not necessarily the same. Forexample, timing of starting the initialization period may be differentfor every pixel, or every group of pixels belonging to the same line, orthe like.

Next, in the writing period, a video signal having image data is inputto the pixel.

In the case where an image is displayed on the entire pixel portion, inone frame period, a selection signal in which a pulse of voltage isshifted is sequentially input to all of the scan lines. Then, in oneline period in which a pulse appears in a selection signal, a videosignal having image data is input to all of the signal lines.

The white pigment and the black pigment in the microcapsule 712 aremoved to the pixel electrode 710 side and the counter electrode 711 sidein accordance with the voltage of the video signal applied to the pixelelectrode 710, so that the display element 705 displays a grayscale.

Also in the writing period, the voltage of a video signal is preferablyapplied to the pixel electrode 710 plural times as in the initializationperiod. Accordingly, the following operation sequence is performedplural times: the video signal is input to a pixel of a line includingthe scan line in a period during which a pulse of a selection signal issupplied to each scan line.

Next, in the holding period, the common voltage Vcom is input to all ofthe pixels through signal lines, and after that, a selection signal isnot input to a scan line or a video signal is not input to a signalline. Accordingly, the positions of the white pigment and the blackpigment in the microcapsule 712 included in the display element 705 aremaintained unless positive or negative voltage is applied between thepixel electrode 710 and the common electrode 711, so that the grayscalelevel displayed on the display element 705 is held. Therefore, an imagewritten in the writing period is maintained even in the holding period.

Note that voltage needed for changing the grayscale level of the displayelement used for electronic paper tends to be higher than that of aliquid crystal element used for a liquid crystal display device or thatof a light-emitting element such as an organic light-emitting elementused for a light-emitting device. Therefore, the potential differencebetween the source electrode and the drain electrode of the transistor704 of a pixel, which serves as a switching element in a writing periodis large; as a result, off-state current is increased, and disturbanceof display is likely to occur due to fluctuation of the potential of thepixel electrode 710. It is effective to increase the capacity of thestorage capacitor 706 in order to prevent the potential of the pixelelectrode 710 from fluctuating due to the off-state current of thetransistor 704. In addition, not only voltage generated between thepixel electrode 710 and the counter electrode 711 but also voltagegenerated between the signal line 707 and the counter electrode 711 isapplied to the microcapsule 712, so that distorted images are displayedby the display element 705 in some cases. In order to prevent generationof this distorted image, it is effective to have a large area of thepixel electrode 710 and to prevent application of voltage which isgenerated between the signal line 707 and the counter electrode 711 tothe microcapsule 712. However, as described above, when the capacity ofthe storage capacitor 706 increases in order to prevent fluctuation ofthe potential of the pixel electrode 710 or the area of the pixelelectrode 710 increases in order to prevent generation of distortedimages on the display, a current value which is to be supplied to thepixel in the writing period increases, so that it takes time to input avideo signal. In an electric paper according to one embodiment of thepresent invention, the crystal region included in the oxidesemiconductor film is in contact with the source electrode or the drainelectrode in the transistor 704 used as a switching element in thepixel, so that the contact resistance between the oxide semiconductorfilm and the source electrode or the drain electrode is decreased andthe on-state current and the field-effect mobility can be increased.Accordingly, even when the capacity of the storage capacitor 706increases or even when the area of the pixel electrode 710 increases, avideo signal can be input to the pixel quickly. Therefore, the length ofthe writing period can be suppressed, and displayed images can beswitched smoothly.

Further, in one embodiment of the present invention, an oxidesemiconductor film whose impurity concentration is extremely low is usedfor an active layer of the transistor 704. Therefore, the off-statecurrent of the transistor 704 in the state where voltage between thegate electrode and the source electrode is substantially zero, that is,leakage current is considerably low. Therefore, even when a potentialdifference between the source electrode and the drain electrode of thetransistor 704 increases in the writing period, off-state current can besuppressed, and generation of disturbance of display due to the changein the potential of the pixel electrode 710 can be prevented. Inaddition, the potential difference between the source electrode and thedrain electrode of the transistor 704 used as a switching element in thepixel increases in the writing period, so that the transistor 704 easilydeteriorates. However, in one embodiment of the present invention,variation in threshold voltage of the transistor 704 due to degradationover time can be reduced, so that reliability of the electronic papercan be enhanced.

This embodiment can be implemented by being combined as appropriate withthe above-described embodiments.

Embodiment 5

FIG. 14A illustrate an example of a block diagram of an active matrixsemiconductor display device. Over a substrate 5300 in the displaydevice, a pixel portion 5301, a first scan line driver circuit 5302, asecond scan line driver circuit 5303, and a signal line driver circuit5304 are provided. In the pixel portion 5301, a plurality of signallines which is extended from the signal line driver circuit 5304 isprovided and a plurality of scan lines which is extended from the firstscan line driver circuit 5302 and the second scan line driver circuit5303 is provided. Pixels which include display elements are provided ina matrix form in respective regions where the scan lines and the signallines intersect with each other. Further, the substrate 5300 in thedisplay device is connected to a timing control circuit 5305 (alsoreferred to as a controller or a controller IC) through a connectionpoint such as a flexible printed circuit (FPC).

In FIG. 14A, the first scan line driver circuit 5302, the second scanline driver circuit 5303, and the signal line driver circuit 5304 areformed over one substrate 5300 provided with the pixel portion 5301.Therefore, since the number of components provided outside, such as adriver circuit is reduced, it is possible not only to downsize thedisplay device but also to reduce cost owing to the decrease in thenumber of assembly steps and inspection steps. In addition, if thedriver circuit is provided outside the substrate 5300, a wiring wouldneed to be extended and the number of wiring connections would beincreased; therefore, the driver circuit is provided over the substrate5300, so that the number of connections of the wirings can be reduced.Therefore, the decrease in yield due to defective connection of thedriver circuit and the pixel portion can be prevented, and the decreasein reliability due to low mechanical strength at a connection point canbe prevented.

Note that as an example, the timing control circuit 5305 supplies afirst scan line driver circuit start signal (GSP1) and a scan linedriver circuit clock signal (GCK1) to the first scan line driver circuit5302. The timing control circuit 5305 supplies, for example, a secondscan line driver circuit start signal (GSP2) (also referred to as astart pulse) and a scan line driver circuit clock signal (GCK2) to thesecond scan line driver circuit 5303. Moreover, the timing controlcircuit 5305 supplies a signal line driver circuit start signal (SSP), asignal line driver circuit clock signal (SCK), video signal data (DATA,also simply referred to as a video signal), and a latch signal (LAT) tothe signal line driver circuit 5304. Note that one of the first scanline driver circuit 5302 and the second scan line driver circuit 5303can be omitted.

FIG. 14B illustrates a structure in which circuits with low drivingfrequency (for example, the first scan line driver circuit 5302 and thesecond scan line driver circuit 5303) are formed over one substrate 5300provided with the pixel portion 5301 and the signal line driver circuit5304 is formed over another substrate which is different from thesubstrate provided with the pixel portion 5301. It is possible to form acircuit with low driving frequency such as an analog switch used for asampling circuit in the signal line driver circuit 5304 partly over onesubstrate 5300 provided with the pixel portion 5301. Thus, asystem-on-panel is partly employed, so that advantages of thesystem-on-panel such as the above-described prevention of decrease inyield due to the defective connection or decrease in mechanical strengthat a connection point, and reduction in cost due to the decrease in thenumber of assembly steps and inspection steps can be obtained more orless. Further, as compared with the system-on-panel in which the pixelportion 5301, the first scan line driver circuit 5302, the second scanline driver circuit 5303, and the signal line driver circuit 5304 areformed over one substrate, it is possible to increase the performance ofa circuit with high driving frequency, and it is possible to form apixel portion having a large area, which is difficult to realize in thecase of using a single crystal semiconductor.

Next, a structure of a signal line driver circuit using an n-channeltransistor is described below.

A signal line driver circuit illustrated in FIG. 15A includes a shiftregister 5601 and a sampling circuit 5602. The sampling circuit 5602includes a plurality of switching circuits 5602_1 to 5602_N (N is anatural number). The switching circuits 5602_1 to 5602_N each include aplurality of n-channel transistors 5603_1 to 5603_k (k is a naturalnumber).

A connection relation in the signal line driver circuit is describedtaking the switching circuit 5602_1 as an example. Note that one of asource electrode and a drain electrode of a transistor is referred to asa first terminal, and the other is referred to as a second terminalbelow.

First terminals of the transistors 5603_1 to 5603_k are connected torespective wirings 5604_1 to 5604_k. A video signal is input to each ofthe wirings 5604_1 to 5604_k. Second terminals of the transistors 5603_1to 5603_k are connected to respective signal lines S1 to Sk. Gateelectrodes of the transistors 5603_1 to 5603_k are connected to theshift register 5601.

The shift register 5601 has the function of sequentially selecting theswitching circuits 5602_1 to 5602_N by sequentially outputting timingsignals having high level (H-level) voltage to wirings 5605_1 to 5605_N.

The switching circuit 5602_1 has a function of controlling respectiveconduction states between the wirings 5604_1 to 5604_k and the signallines S1 to Sk (conduction between the first terminal and the secondterminal), namely a function of controlling whether or not to supplyrespective potentials of the wirings 5604_1 to 5604_k to the signallines S1 to Sk with the use of switching of the transistors 5603_1 to5603_k.

Next, the operation of the signal line driver circuit in FIG. 15A isdescribed with reference to a timing chart in FIG. 15B. FIG. 15Billustrates the timing chart of the timing signals Sout_1 to Sout_Nwhich are input to the respective wirings 5605_1 to 5605_N and videosignals Vdata_1 to Vdata_k which are input to the respective wirings5604_1 to 5604_k from the shift register 5601, as an example.

Note that one operation period of the signal line driver circuitcorresponds to one line period in a display device. Illustrated in FIG.15B is the case where one line period is divided into periods T1 to TN.The periods T1 to TN are each periods for writing a video signal intoone pixel in a selected row.

In the periods T1 to TN, the shift register 5601 sequentially outputsH-level timing signals to the wirings 5605_1 to 5605_N. For example, inthe period T1, the shift register 5601 outputs an H level signal to thewiring 5605_1. Accordingly, the transistors 5603_1 to 5603_k included inthe switching circuit 5602_1 are turned on, so that the wirings 5604_1to 5604_k and the signal lines S1 to Sk are electrically connected toeach other. In that case, Data(S1) to Data(Sk) are input to therespective wirings 5604_1 to 5604_k. The Data(S1) to Data(Sk) are inputto respective pixels in the first to kth columns in the selected rowthrough the respective transistors 5603_1 to 5603_k. Thus, in theperiods T1 to TN, video signals are sequentially written to the pixelsin the selected row by k columns.

By writing video signals to the pixels by a plurality of columns, thenumber of video signals or the number of wirings can be reduced. Thus,the number of connections with an external circuit such as a controllercan be reduced. By writing video signals to the pixels per pluralcolumns, writing time can be extended and lack of writing of videosignals can be prevented.

Next, one embodiment of a shift register which is used for a signal linedriver circuit or a scan line driver circuit is described with referenceto FIGS. 16A and 16B and FIGS. 17A and 17B.

The shift register includes first to N-th pulse output circuits 10_1 to10_N (N is a natural number of 3 or more) (see FIG. 16A). A first clocksignal CK1, a second clock signal CK2, a third clock signal CK3, and afourth clock signal CK4 are supplied from a first wiring 11, a secondwiring 12, a third wiring 13, and a fourth wiring 14, respectively, tothe first to N-th pulse output circuits 10_1 to 10_N. A start pulse SP1(a first start pulse) from a fifth wiring 15 is input to the first pulseoutput circuit 10_1. Further, a signal from the pulse output circuit10_(n−1) of the previous stage (referred to as a previous stage signalOUT(n−1)) (n is a natural number of 2 or more) is input to the n-thpulse output circuit 10_n (n is a natural number of 2 or more and N orless) in a second or subsequent stage. A signal from the third pulseoutput circuit 10_3 which is two stages after the first pulse outputcircuit 10_1 is input to the first pulse output circuit 10_1. In asimilar way, a signal from the (n+2)-th pulse output circuit 10_(n+2)which is two stages after the n-th pulse output circuit 10_n (referredto as the subsequent stage signal OUT(n+2)) is input to the n-th pulseoutput circuit 10_n in the second stage or subsequent stage. Thus, thepulse output circuits of the respective stages output first outputsignals (OUT(1)(SR) to OUT(N)(SR)) to be input to the pulse outputcircuits of their respective subsequent stages and/or to the pulseoutput circuits of their respective stages before their respectiveprevious stages, and second output signals (OUT(1) to OUT(N)) to beinput to another circuit or the like. Note that as illustrated in FIG.16A, a subsequent stage signal OUT(n+2) is not input to the last twostages of the shift register; therefore, as an example, a second startpulse SP2 and a third start pulse SP3 may be input thereto.

Note that the clock signal (CK) is a signal which alternates between anH level and an L level (low level voltage) at regular intervals. Thefirst to fourth clock signals (CK1) to (CK4) are delayed by a ¼ periodsequentially. In this embodiment, by using the first to fourth clocksignals (CK1) to (CK4), control or the like of driving of a pulse outputcircuit is performed.

A first input terminal 21, a second input terminal 22, and a third inputterminal 23 are electrically connected to any of the first to fourthwirings 11 to 14. For example, in FIG. 16A, the first input terminal 21of the first pulse output circuit 10_1 is electrically connected to thefirst wiring 11, the second input terminal 22 of the first pulse outputcircuit 10_1 is electrically connected to the second wiring 12, and thethird input terminal 23 of the first pulse output circuit 10_1 iselectrically connected to the third wiring 13. In addition, the firstinput terminal 21 of the second pulse output circuit 10_2 iselectrically connected to the second wiring 12, the second inputterminal 22 of the second pulse output circuit 10_2 is electricallyconnected to the third wiring 13, and the third input terminal 23 of thesecond pulse output circuit 10_2 is electrically connected to the fourthwiring 14.

Each of the first to N-th pulse output circuits 10_1 to 10_N includesthe first input terminal 21, the second input terminal 22, the thirdinput terminal 23, a fourth input terminal 24, a fifth input terminal25, a first output terminal 26, and a second output terminal 27 (seeFIG. 16B). In the first pulse output circuit 10_1, the first clocksignal CK1 is input to the first input terminal 21; the second clocksignal CK2 is input to the second input terminal 22; the third clocksignal CK3 is input to the third input terminal 23; the start pulse isinput to the fourth input terminal 24; the subsequent stage signalOUT(3) is input to the fifth input terminal 25; the first output signalOUT(1)(SR) is output from the first output terminal 26; and the secondoutput signal OUT(1) is output from the second output terminal 27.

Next, an example of a specific circuit structure of a pulse outputcircuit will be described with reference to FIG. 17A.

Pulse output circuits each include 1st to 13th transistors 31 to 43 (seeFIG. 17A). Signals or power supply potentials are supplied to the 1st to13th transistors 31 to 43 from a power supply line 51 to which a firsthigh power supply potential VDD is supplied, a power supply line 52 towhich a second high power supply potential VCC is supplied, and a powersupply line 53 to which a low power supply potential VSS is supplied, inaddition to the above-described first to fifth input terminals 21 to 25,the first output terminal 26, and the second output terminal 27. Therelation of the power supply potentials of the power supply lines inFIG. 17A is as follows: the first power supply potential VDD is higherthan or equal to the second power supply potential VCC, and the secondpower supply potential VCC is higher than the third power supplypotential VSS. The first to fourth clock signals (CK1) to (CK4) aresignals which become H-level signals and L-level signals repeatedly atregular intervals. The potential is VDD when the clock signal is at theH level, and the potential is VSS when the clock signal is at the Llevel. By making the potential VDD of the power supply line 51 higherthan the potential VCC of the power supply line 52, a potential appliedto a gate electrode of a transistor can be lowered, shift in thethreshold voltage of the transistor can be reduced, and deterioration ofthe transistor can be suppressed without an adverse effect on theoperation of the transistor.

In FIG. 17A, a first terminal of the 1st transistor 31 is electricallyconnected to the power supply line 51, a second terminal of the 1sttransistor 31 is electrically connected to a first terminal of the 9thtransistor 39, and a gate electrode of the 1st transistor 31 iselectrically connected to the fourth input terminal 24. A first terminalof the 2nd transistor 32 is electrically connected to the power supplyline 53, a second terminal of the 2nd transistor 32 is electricallyconnected to the first terminal of the 9th transistor 39, and a gateelectrode of the 2nd transistor 32 is electrically connected to a gateelectrode of the 4th transistor 34. A first terminal of the 3rdtransistor 33 is electrically connected to the first input terminal 21,and a second terminal of the 3rd transistor 33 is electrically connectedto the first output terminal 26. A first terminal of the 4th transistor34 is electrically connected to the power supply line 53, and a secondterminal of the 4th transistor 34 is electrically connected to the firstoutput terminal 26. A first terminal of the 5th transistor 35 iselectrically connected to the power supply line 53, a second terminal ofthe 5th transistor 35 is electrically connected to the gate electrode ofthe 2nd transistor 32 and the gate electrode of the 4th transistor 34,and a gate electrode of the 5th transistor 35 is electrically connectedto the fourth input terminal 24. A first terminal of the 6th transistor36 is electrically connected to the power supply line 52, a secondterminal of the 6th transistor 36 is electrically connected to the gateelectrode of the 2nd transistor 32 and the gate electrode of the 4thtransistor 34, and a gate electrode of the 6th transistor 36 iselectrically connected to the fifth input terminal 25. A first terminalof the 7th transistor 37 is electrically connected to the power supplyline 52, a second terminal of the 7th transistor 37 is electricallyconnected to a second terminal of the 8th transistor 38, and a gateelectrode of the 7th transistor 37 is electrically connected to thethird input terminal 23. A first terminal of the 8th transistor 38 iselectrically connected to the gate electrode of the 2nd transistor 32and the gate electrode of the 4th transistor 34, and a gate electrode ofthe 8th transistor 38 is electrically connected to the second inputterminal 22. The first terminal of the 9th transistor 39 is electricallyconnected to the second terminal of the 1st transistor 31 and the secondterminal of the 2nd transistor 32, a second terminal of the 9thtransistor 39 is electrically connected to a gate electrode of the 3rdtransistor 33 and a gate electrode of the 10th transistor 40, and a gateelectrode of the 9th transistor 39 is electrically connected to thepower supply line 52. A first terminal of the 10th transistor 40 iselectrically connected to the first input terminal 21, a second terminalof the 10th transistor 40 is electrically connected to the second outputterminal 27, and the gate electrode of the 10th transistor 40 iselectrically connected to the second terminal of the 9th transistor 39.A first terminal of the 11th transistor 41 is electrically connected tothe power supply line 53, a second terminal of the 11th transistor 41 iselectrically connected to the second output terminal 27, and a gateelectrode of the 11th transistor 41 is electrically connected to thegate electrode of the 2nd transistor 32 and the gate electrode of the4th transistor 34. A first terminal of the 12th transistor 42 iselectrically connected to the power supply line 53, a second terminal ofthe 12th transistor 42 is electrically connected to the second outputterminal 27, and a gate electrode of the 12th transistor 42 iselectrically connected to the gate electrode of the 7th transistor 37. Afirst terminal of the 13th transistor 43 is electrically connected tothe power supply line 53, a second terminal of the 13th transistor 43 iselectrically connected to the first output terminal 26, and a gateelectrode of the 13th transistor 43 is electrically connected to thegate electrode of the 7th transistor 37.

In FIG. 17A, a connection point of the gate electrode of the 3rdtransistor 33, the gate electrode of the 10th transistor 40, and thesecond terminal of the 9th transistor 39 is referred to as a node A. Aconnection point where the gate electrode of the 2nd transistor 32, thegate electrode of the 4th transistor 34, the second terminal of the 5thtransistor 35, the second terminal of the 6th transistor 36, the firstterminal of the 8th transistor 38, and the gate electrode of the 11thtransistor 41 are connected is referred to as a node B (see FIG. 17A).

FIG. 17B is a timing chart of the shift register including a pluralityof pulse output circuits illustrated in FIG. 17A.

The provision of the 9th transistor 39 in which the second power supplypotential VCC is applied to the gate electrode as illustrated in FIG.17A has the following advantages before and after bootstrap operation.

Without the 9th transistor 39 in which the second high power supplypotential VCC is applied to the gate electrode, as the potential of thenode A is raised by the bootstrap operation, the potential of the sourceelectrode which is the second terminal of the 1st transistor 31 rises toa value higher than the first power supply potential VDD. Then, thefirst terminal of the 1st transistor 31, namely the power supply line 51side thereof, becomes to serve as the source electrode. Consequently, inthe 1st transistor 31, high bias voltage is applied and thus significantstress is applied between the gate electrode and the source electrodeand between the gate electrode and the drain electrode, which mightcause deterioration of the transistor. By providing of the 9thtransistor 39 in which the second power supply potential VCC is appliedto the gate electrode, an increase in the potential of the secondterminal of the 1st transistor 31 can be prevented, though the potentialof the node A is raised by the bootstrap operation. In other words, theprovision of the 9th transistor 39 can lower the level of negative biasvoltage applied between the gate electrode and the source electrode ofthe 1st transistor 31. Accordingly, with a circuit structure in thisembodiment, negative bias voltage applied between the gate electrode andthe source electrode of the 1st transistor 31 can be lowered, so thatdeterioration of the 1st transistor 31, which is due to stress, can besuppressed.

The 9th transistor 39 is provided so as to be connected between thesecond terminal of the 1st transistor 31 and the gate electrode of the3rd transistor 33 through the first terminal and the second terminalthereof. Note that when the shift register including a plurality ofpulse output circuits in this embodiment is included in a signal linedriver circuit having a larger number of stages than a scan line drivercircuit, the 9th transistor 39 may be omitted, which is advantageous inthat the number of transistors can be reduced.

Note that an oxide semiconductor is used for active layers of the 1st to13th transistors 31 to 43; thus, the off-state current of thetransistors can be reduced, the on-state current and field effectmobility can be increased, and the degree of deterioration of thetransistors can be reduced; thus, a malfunction in a circuit can bereduced. Further, the degree of deterioration of the transistor formedusing an oxide semiconductor by application of a high potential to agate electrode is smaller than that of a transistor formed usingamorphous silicon. Therefore, even when the first power supply potentialVDD is supplied to a power supply line to which the second power supplypotential VCC is supplied, similar operation can be performed, and thenumber of power supply lines which are provided for a circuit can bereduced, so that the circuit can be miniaturized.

A similar function to the above is obtained even when the connectionrelation is changed so that a clock signal that is supplied to the gateelectrode of the 7th transistor 37 from the third input terminal 23 anda clock signal that is supplied to the gate electrode of the 8thtransistor 38 from the second input terminal 22 may be supplied from thesecond input terminal 22 and the third input terminal 23, respectively.Note that in the shift register shown in FIG. 17A, if the state wherethe 7th transistor 37 and the 8th transistor 38 are both on is changedthrough the state where the 7th transistor 37 is off and the 8thtransistor 38 is on to the state where the 7th transistor 37 is off andthe 8th transistor 38 is off, potential reduction at the node B, whichis caused by potential reduction of the second input terminal 22 and thethird input terminal 23, is caused twice due to the potential reductionof the gate electrode of the 7th transistor 37 and the potentialreduction of the gate electrode of the 8th transistor 38. On the otherhand, in the case where states of the 7th transistor 37 and the 8thtransistor 38 in the shift register illustrated in FIG. 17A are changedin such a manner that both the 7th transistor 37 and the 8th transistor38 are on, then the 7th transistor 37 is on and the 8th transistor 38 isoff, and then the 7th transistor 37 and the 8th transistor 38 are off,the potential reduction at the node B, which is caused by the potentialreduction of the second input terminal 22 and the third input terminal23, is caused only once by the potential reduction of the gate electrodeof the 8th transistor 38. Consequently, a connection relation in whichthe clock signal CK3 is supplied from the third input terminal 23 to thegate electrode of the 7th transistor 37 and the clock signal CK2 issupplied from the second input terminal 22 to the gate electrode of the8th transistor 38 is preferable. This is because the number of times ofthe change in the potential of the node B can be reduced, and the noisecan be decreased.

In this way, in a period during which the potentials of the first outputterminal 26 and the second output terminal 27 are held at the L level,the H level signal is regularly supplied to the node B, wherebymalfunction of a pulse output circuit can be suppressed.

This embodiment can be implemented by being combined as appropriate withthe above-described embodiments.

Embodiment 6

In a liquid crystal display device according to one embodiment of thepresent invention, a highly reliable transistor with low off-statecurrent is used; therefore, high visibility and high reliability areobtained. In Embodiment 6, a structure of the liquid crystal displaydevice according to one embodiment of the present invention will bedescribed.

FIG. 18 is a cross-sectional view of a pixel of a liquid crystal displaydevice according to one embodiment of the present invention, as anexample. A transistor 1401 illustrated in FIG. 18 has a gate electrode1402 formed over an insulating surface, a gate insulating film 1403 overthe gate electrode 1402, an oxide semiconductor film 1404 which is overthe gate insulating film 1403 and which overlaps with the gate electrode1402, and a pair of conductive films 1406 a and 1406 b which function asa source electrode and a drain electrode and which are stacked over theoxide semiconductor film 1404. Further, the transistor 1401 may includean insulating film 1407 formed over the oxide semiconductor film 1404 asa component. The insulating film 1407 is formed so as to cover the gateelectrode 1402, the gate insulating film 1403, the oxide semiconductorfilm 1404, and the conductive films 1406 a and 1406 b. The oxidesemiconductor film 1404 includes an amorphous region 1430 and a crystalregion 1431 over the amorphous region 1430. The crystal region 1431 isin contact with the conductive films 1406 a and 1406 b.

An insulating film 1408 is formed over the insulating film 1407. Part ofthe insulating film 1407 and the insulating film 1408 is provided withan opening, and a pixel electrode 1410 is formed so as to be in contactwith the conductive film 1406 b in the opening.

Further, a spacer 1417 for controlling a cell gap of a liquid crystalelement is formed over the insulating film 1408. The spacer 1417 can beformed by etching an insulating film into an appropriate shape.Alternatively, the cell gap may be controlled by dispersing a fillerover the insulating film 1408.

An alignment film 1411 is formed over the pixel electrode 1410. Further,a counter electrode 1413 is provided in a position opposed to the pixelelectrode 1410, and an alignment film 1414 is formed on the side of thecounter electrode 1413 which is close to the pixel electrode 1410. Thealignment film 1411 and the alignment film 1414 can be formed using anorganic resin such as polyimide or polyvinyl alcohol. Alignmenttreatment such as rubbing is performed on their surfaces in order toalign liquid crystal molecules in certain direction. Rubbing can beperformed by rolling a roller wrapped with cloth of nylon or the likewhile applying pressure on the alignment film so that the surface of thealignment film is rubbed in certain direction. Note that it is alsopossible to form the alignment films 1411 and 1414 that have alignmentcharacteristics by using an inorganic material such as silicon oxide byan evaporation method, without alignment process. Furthermore, a liquidcrystal 1415 is provided in a region which is surrounded by a sealant1416 between the pixel electrode 1410 and the counter electrode 1413.Injection of the liquid crystal 1415 may be performed by a dispensermethod (dripping method) or a dipping method (pumping method). A fillermay be mixed in the sealant 1416.

The liquid crystal element formed using the pixel electrode 1410, thecounter electrode 1413, and the liquid crystal 1415 may overlap with acolor filter through which light in a particular wavelength region canpass. The color filter may be formed on a substrate (counter substrate)1420 provided with the counter electrode 1413. The color filter can beselectively formed by photolithography after application of an organicresin such as an acrylic-based resin in which a pigment is dispersed onthe substrate 1420. Alternatively, the color filter can be selectivelyformed by etching after application of a polyimide-based resin in whicha pigment is dispersed on the substrate 1420. Further alternatively, thecolor filter can be selectively formed by a droplet discharge methodsuch as an ink jet method.

A light-blocking film which can block light may be formed between thepixels so that disclination due to variations between the pixels in thealignment of the liquid crystal 1415 is prevented from seeing. Thelight-blocking film can be formed using an organic resin containing ablack pigment such as a carbon black or a low-valent titanium oxide.Alternatively, the light-blocking film may be formed using a film usingchromium.

The pixel electrode 1410 and the counter electrode 1413 can be formedusing a transparent conductive material such as indium tin oxideincluding silicon oxide (ITSO), indium tin oxide (ITO), zinc oxide(ZnO), indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO), forexample. Although an example of manufacturing a transmissive liquidcrystal element by using a light-transmitting conductive film for thepixel electrode 1410 and the counter electrode 1413 is described inEmbodiment 6, one embodiment of the present invention is not limited tothis structure. The liquid crystal display device according to oneembodiment of the present invention may be a transreflective liquidcrystal display device or a reflective liquid crystal display device.

Although a liquid crystal display device of a TN (twisted nematic) modeis described in Embodiment 6, the transistor of the present inventioncan be used for other liquid crystal display devices of a VA (verticalalignment) mode, an OCB (optically compensated birefringence) mode, anIPS (in-plane-switching) mode, and the like.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase as temperature of cholesteric liquidcrystal is increased. Since the blue phase is generated within an onlynarrow range of temperature, a liquid crystal composition containing achiral agent at 5 wt % or more so as to improve the temperature range isused for the liquid crystal 1415. The liquid crystal compositionincluding liquid crystal exhibiting a blue phase and a chiral agent hasa short response time of greater than or equal to 10 μsec and less thanor equal to 100 μsec and is optically isotropic; therefore, alignmenttreatment is not necessary and viewing angle dependence is small.

FIG. 19 is an example of a perspective view showing a structure of aliquid crystal display device of the present invention. The liquidcrystal display device illustrated in FIG. 19 is provided with a liquidcrystal panel 1601 in which a liquid crystal element is formed between apair of substrates; a first diffusion plate 1602; a prism sheet 1603; asecond diffusion plate 1604; a light guide plate 1605; a reflectionplate 1606; a light source 1607; and a circuit board 1608.

The liquid crystal panel 1601, the first diffusion plate 1602, the prismsheet 1603, the second diffusion plate 1604, the light guide plate 1605,and the reflection plate 1606 are sequentially stacked. The light source1607 is provided at an end portion of the light guide plate 1605. Theliquid crystal panel 1601 is uniformly irradiated with light from thelight source 1607 which is diffused inside the light guide plate 1605,due to the first diffusion plate 1602, the prism sheet 1603, and thesecond diffusion plate 1604.

Although the first diffusion plate 1602 and the second diffusion plate1604 are used in this embodiment, the number of diffusion plates is notlimited thereto. The number of diffusion plates may be one, or may bethree or more. It is acceptable as long as the diffusion plate isprovided between the light guide plate 1605 and the liquid crystal panel1601. Therefore, a diffusion plate may be provided only on the sidecloser to the liquid crystal panel 1601 than the prism sheet 1603, ormay be provided only on the side closer to the light guide plate 1605than the prism sheet 1603.

Further, the cross section of the prism sheet 1603 is not limited to asawtooth-shape illustrated in FIG. 19 . The prism sheet 1603 may haveany shape as long as light from the light guide plate 1605 can beconcentrated on the liquid crystal panel 1601 side.

The circuit board 1608 is provided with a circuit which generatesvarious kinds of signals input to the liquid crystal panel 1601, acircuit which processes the signals, or the like. In FIG. 19 , thecircuit board 1608 and the liquid crystal panel 1601 are connected toeach other via a flexible printed circuit (FPC) 1609. Note that thecircuit may be connected to the liquid crystal panel 1601 by using achip on glass (COG) method, or part of the circuit may be connected tothe FPC 1609 by using a chip on film (COF) method.

FIG. 19 illustrates an example in which the circuit board 1608 isprovided with a control circuit which controls driving of the lightsource 1607 and the control circuit and the light source 1607 areconnected to each other via an FPC 1610. Note that the above-describedcontrol circuit may be formed over the liquid crystal panel 1601. Inthat case, the liquid crystal panel 1601 and the light source 1607 areconnected to each other via an FPC or the like.

Although FIG. 19 illustrates an edge-light type light source in whichthe light source 1607 is provided at an end portion of the liquidcrystal panel 1601, a liquid crystal display device of the presentinvention may be a direct-below type in which the light source 1607 isprovided directly below the liquid crystal panel 1601.

This embodiment can be implemented by being combined as appropriate withthe above-described embodiments.

Embodiment 7

In Embodiment 7, a structure of a light-emitting device using thetransistor according to one embodiment of the present invention for apixel will be described. In this embodiment, a cross-sectional structureof a pixel when a transistor for driving a light-emitting element is ann-channel transistor is described with reference to FIG. 20A to 20C.Although the case where a first electrode is a cathode and a secondelectrode is an anode is described in FIGS. 20A to 20C, the firstelectrode may be an anode and the second electrode may be a cathode.

FIG. 20A is a cross-sectional view of a pixel in the case where ann-channel transistor is employed as a transistor 6031, and light emittedfrom a light-emitting element 6033 is extracted from a first electrode6034 side. The transistor 6031 is covered with an insulating film 6037,and a partition 6038 having an opening is formed over the insulatingfilm 6037. In the opening of the partition 6038, the first electrode6034 is partly exposed, and the first electrode 6034, anelectroluminescent layer 6035, and a second electrode 6036 aresequentially stacked in the opening.

The first electrode 6034 is formed using a material or to a thicknesssuch that light transmits therethrough, and can be formed using amaterial having a low work function of a metal, an alloy, anelectrically conductive compound, a mixture thereof, or the like.Specifically, an alkali metal such as Li or Cs, an alkaline earth metalsuch as Mg, Ca, or Sr, an alloy containing such metals (e.g., Mg:Ag,Al:Li, or Mg:In), a compound of such materials (e.g., calcium fluorideor calcium nitride), or a rare-earth metal such as Yb or Er can be used.Further, in the case where an electron injection layer is provided,another conductive layer such as an aluminum layer may be used. Then,the first electrode 6034 is formed to a thickness such that lighttransmits therethrough (preferably, about 5 nm to 30 nm). Furthermore,the sheet resistance of the first electrode 6034 may be suppressed byformation of a light-transmitting conductive layer of alight-transmitting oxide conductive material so as to be in contact withand over or under the above-described conductive layer with a thicknesssuch that light transmits therethrough. Alternatively, the firstelectrode 6034 may be formed using only a conductive layer of anotherlight-transmitting oxide conductive material such as indium tin oxide(ITO), zinc oxide (ZnO), indium zinc oxide (IZO), or gallium-doped zincoxide (GZO). Furthermore, a mixture in which zinc oxide (ZnO) is mixedat 2% to 20% in ITSO or in indium oxide including silicon oxide may beused. In the case of using the light-transmitting oxide conductivematerial, it is preferable to provide an electron injection layer in theelectroluminescent layer 6035.

The second electrode 6036 is formed using a material and to a thicknesssuch that light is reflected or blocked, and can be formed using amaterial suitable for being used as an anode. For example, asingle-layer film including one or more of titanium nitride, zirconiumnitride, titanium, tungsten, nickel, platinum, chromium, silver,aluminum, and the like, a stacked layer of a titanium nitride filmcontaining titanium nitride as a main component and a film containingaluminum as a main component, a three-layer structure of a titaniumnitride film, a film containing aluminum as a main component, and atitanium nitride film, or the like can be used for the second electrode6036.

The electroluminescent layer 6035 is formed using a single layer or aplurality of layers. When the electroluminescent layer 6035 is formedwith a plurality of layers, these layers can be classified into a holeinjection layer, a hole transport layer, a light-emitting layer, anelectron transport layer, an electron injection layer, and the like inview of the carrier transport property. In the case where theelectroluminescent layer 6035 includes at least one of a hole injectionlayer, a hole transport layer, an electron transport layer, and anelectron injection layer in addition to a light-emitting layer, theelectron injection layer, the electron transport layer, thelight-emitting layer, the hole transport layer, and the hole injectionlayer are sequentially stacked over the first electrode 6034. Note thatthe boundary between layers which are adjacent to each other is notnecessarily clear, and there may be the case where the boundary isunclear since materials for forming the layers are mixed. Each layer maybe formed with an organic material or an inorganic material. As theorganic material, any of a high molecular weight material, a mediummolecular weight material, and a low molecular weight material may beused. Note that the medium molecular weight material corresponds to alow polymer in which the number of repetitions of a structural unit (thedegree of polymerization) is about 2 to 20. A distinction between a holeinjection layer and a hole transport layer is not always distinct, andthey are the same in the sense that a hole transport property (holemobility) is a particularly important characteristic. A layer being incontact with the anode is referred to as a hole injection layer and alayer being in contact with the hole injection layer is referred to as ahole transport layer for convenience. The same is also true for theelectron transport layer and the electron injection layer; a layer beingin contact with the cathode is referred to as an electron injectionlayer and a layer being in contact with the electron injection layer isreferred to as an electron transport layer. In some cases, thelight-emitting layer also functions as the electron transport layer, andit is therefore referred to as a light-emitting electron transportlayer, too.

In the case of the pixel illustrated in FIG. 20A, light emitted from thelight-emitting element 6033 can be extracted from the first electrode6034 side as shown by a hollow arrow.

Next, a cross-sectional view of a pixel in the case where an n-channeltransistor is used as a transistor 6041, and light emitted from alight-emitting element 6043 is extracted from a second electrode 6046side is illustrated in FIG. 20B. The transistor 6041 is covered with aninsulating film 6047, and a partition 6048 having an opening is formedover the insulating film 6047. In the opening of the partition 6048, afirst electrode 6044 is partly exposed, and the first electrode 6044, anelectroluminescent layer 6045, and the second electrode 6046 aresequentially stacked in the opening.

The first electrode 6044 is formed using a material and to a thicknesssuch that light is reflected or blocked, and can be formed using amaterial having a low work function of a metal, an alloy, anelectrically conductive compound, a mixture thereof, or the like.Specifically, an alkali metal such as Li or Cs, an alkaline earth metalsuch as Mg, Ca, or Sr, an alloy containing such metals (e.g., Mg:Ag,Al:Li, or Mg:In), a compound of such materials (e.g., calcium fluorideor calcium nitride), or a rare-earth metal such as Yb or Er can be used.Further, in the case where an electron injection layer is provided,another conductive film such as an aluminum film may be used.

The second electrode 6046 is formed using a material or to a thicknesssuch that light transmits therethrough, and formed using a materialsuitable for being used as an anode. For example, anotherlight-transmitting oxide conductive material such as indium tin oxide(ITO), zinc oxide (ZnO), indium zinc oxide (IZO), or gallium-doped zincoxide (GZO) can be used for the second electrode 6046. Further, amixture in which zinc oxide (ZnO) is mixed at 2% to 20% in ITSO orindium oxide including silicon oxide may be used for the secondelectrode 6046. Furthermore, other than the above-describedlight-transmitting oxide conductive material, a single-layer filmincluding one or more of titanium nitride, zirconium nitride, titanium,tungsten, nickel, platinum, chromium, silver, aluminum, and the like, astacked layer of a film containing titanium nitride as a main componentand a film containing aluminum as a main component, a three-layerstructure of a titanium nitride film, a film containing aluminum as amain component, and a titanium nitride film, or the like can be used forthe second electrode 6046. However, in the case of using a materialother than the light-transmitting oxide conductive material, the secondelectrode 6046 is formed to have a thickness such that light transmitstherethrough (preferably, about 5 nm to 30 nm).

The electroluminescent layer 6045 can be formed in a manner similar tothe electroluminescent layer 6035 of FIG. 20A.

In the case of the pixel illustrated in FIG. 20B, light emitted from thelight-emitting element 6043 can be extracted from the second electrode6046 side as shown by a hollow arrow.

Next, a cross-sectional view of a pixel in the case where an n-channeltransistor is used as a transistor 6051, and light emitted from alight-emitting element 6053 is extracted from a first electrode 6054side and a second electrode 6056 side is illustrated in FIG. 20C. Thetransistor 6051 is covered with an insulating film 6057, and a partition6058 having an opening is formed over the insulating film 6057. In theopening of the partition 6058, the first electrode 6054 is partlyexposed, and the first electrode 6054, an electroluminescent layer 6055,and the second electrode 6056 are sequentially stacked in the opening.

The first electrode 6054 can be formed in a manner similar to that ofthe first electrode 6034 in FIG. 20A. The second electrode 6056 can beformed in a manner similar to that of the second electrode 6046 of FIG.20B. The electroluminescent layer 6055 can be formed in a manner similarto that of the electroluminescent layer 6035 of FIG. 20A.

In the case of the pixel illustrated in FIG. 20C, light emitted from thelight-emitting element 6053 can be extracted from both sides of a firstelectrode 6054 side and a second electrode 6056 side as shown by hollowarrows.

This embodiment can be implemented by being combined as appropriate withthe above-described embodiments.

Example 1

A semiconductor device according to one embodiment of the presentinvention can be used so that a highly reliable electronic device, anelectronic device with low power consumption, and an electronic devicewith high-speed driving can be provided. In addition, a semiconductordisplay device according to one embodiment of the present invention canbe used so that a highly reliable electronic device, an electronicdevice with high visibility, and an electronic device with low powerconsumption can be provided. In particular, in the case where a portableelectronic device which has difficulty in continuously receiving power,a semiconductor device or a semiconductor display device with low powerconsumption according to one embodiment of the present invention isadded to the component of the device, whereby an advantage in increasingthe continuous duty period can be obtained. Further, by use of atransistor with low off-state current, redundant circuit design which isneeded to compensate high off-state current is unnecessary; therefore,the density of an integrated circuit used for the semiconductor devicecan be increased, and a higher performance semiconductor device can beformed.

Moreover, with the semiconductor device of the present invention, theheat treatment temperature in the manufacturing process can besuppressed; therefore, a highly reliable thin film transistor withexcellent characteristics can be formed even when the thin filmtransistor is formed over a substrate formed using a flexible syntheticresin of which heat resistance is lower than that of glass, such asplastic.

Accordingly, with the use of the manufacturing method according to oneembodiment of the present invention, a highly reliable, lightweight, andflexible semiconductor device can be provided. Examples of a plasticsubstrate include polyester typified by polyethylene terephthalate(PET), polyethersulfone (PES), polyethylene naphthalate (PEN),polycarbonate (PC), polyetheretherketone (PEEK), polysulfone (PSF),polyetherimide (PEI), polyarylate (PAR), polybutylene terephthalate(PBT), polyimide, an acrylonitrile-butadiene-styrene resin, polyvinylchloride, polypropylene, polyvinyl acetate, an acrylic resin, and thelike.

The semiconductor device according to one embodiment of the presentinvention can be used for display devices, laptops, or image reproducingdevices provided with recording media (typically, devices whichreproduce the content of recording media such as digital versatile discs(DVDs) and have displays for displaying the reproduced images). Otherthan the above, as an electronic device which can be provided with thesemiconductor device according to one embodiment of the presentinvention, mobile phones, portable game machines, portable informationterminals, e-book readers, video cameras, digital still cameras,goggle-type displays (head mounted displays), navigation systems, audioreproducing devices (e.g., car audio systems and digital audio players),copiers, facsimiles, printers, multifunction printers, automated tellermachines (ATM), vending machines, and the like can be given. FIGS. 21Ato 21F illustrate specific examples of these electronic devices.

FIG. 21A illustrates an e-book reader including a housing 7001, adisplay portion 7002, and the like. The semiconductor display deviceaccording to one embodiment of the present invention can be used for thedisplay portion 7002, so that a highly reliable e-book reader, an e-bookreader capable of displaying an image with high visibility, and ane-book reader with low power consumption can be provided. Thesemiconductor device according to one embodiment of the presentinvention can be used for an integrated circuit for controlling drivingof the e-book reader, so that a highly reliable e-book reader, an e-bookreader with low power consumption, an e-book reader with high-speeddriving, and a higher performance e-book reader can be provided. When aflexible substrate is used, a semiconductor device and a semiconductordisplay device can have flexibility, whereby a user-friendly e-bookreader which is flexible and lightweight can be provided.

FIG. 21B illustrates a display device including a housing 7011, adisplay portion 7012, a supporting base 7013, and the like. Thesemiconductor display device according to one embodiment of the presentinvention can be used for the display portion 7012, so that a highlyreliable display device, a display device capable of displaying an imagewith high visibility, and a display device with low power consumptioncan be provided. The semiconductor device according to one embodiment ofthe present invention can be used for an integrated circuit forcontrolling driving of the display device, so that a highly reliabledisplay device, a display device with low power consumption, a displaydevice with high-speed driving, and a higher performance display devicecan be provided. The display device includes in its category any kind ofdisplay device for displaying information, such as display devices forpersonal computers, for receiving television broadcast, and fordisplaying advertisement.

FIG. 21C illustrates a display device including a housing 7021, adisplay portion 7022, and the like. The semiconductor display deviceaccording to one embodiment of the present invention can be used for thedisplay portion 7022, so that a highly reliable display device, adisplay device capable of displaying an image with high visibility, anda display device with low power consumption can be provided. Thesemiconductor device according to one embodiment of the presentinvention can be used for an integrated circuit for controlling drivingof the display device, so that a highly reliable display device, adisplay device with low power consumption, and a higher performancedisplay device can be provided. When a flexible substrate is used, asemiconductor device and a semiconductor display device can haveflexibility, whereby a user-friendly display device which is flexibleand lightweight can be provided. Accordingly, as illustrated in FIG.21C, a display device can be used while being fixed to fabric or thelike, and an application range of the display device is dramaticallywidened.

FIG. 21D illustrates a portable game machine including a housing 7031, ahousing 7032, a display portion 7033, a display portion 7034, amicrophone 7035, speakers 7036, an operation key 7037, a stylus 7038,and the like. The semiconductor display device according to oneembodiment of the present invention can be used for the display portion7033 and the display portion 7034, so that a highly reliable portablegame machine, a portable game machine capable of displaying an imagewith high visibility, and a portable game machine with low powerconsumption can be provided. The semiconductor device according to oneembodiment of the present invention can be used for an integratedcircuit for controlling driving of the portable game machine, so that ahighly reliable portable game machine, a portable game machine with lowpower consumption, and a higher performance portable game machine can beprovided. Although the portable game machine illustrated in FIG. 21Dincludes two display portions 7033 and 7034, the number of displayportions included in the portable game machine is not limited to two.

FIG. 21E illustrates a mobile phone including a housing 7041, a displayportion 7042, an audio input portion 7043, an audio output portion 7044,operation keys 7045, a light-receiving portion 7046, and the like. Lightreceived in the light-receiving portion 7046 is converted intoelectrical signals, whereby an outside image can be downloaded. Thesemiconductor display device according to one embodiment of the presentinvention can be used for the display portion 7042, so that a highlyreliable mobile phone, a mobile phone capable of displaying an imagewith high visibility, and a mobile phone with low power consumption canbe provided. The semiconductor device according to one embodiment of thepresent invention can be used for an integrated circuit for controllingdriving of the mobile phone, so that a highly reliable mobile phone, amobile phone with low power consumption, a mobile phone with high-speeddriving, and a higher performance mobile phone can be provided.

FIG. 21F illustrates a portable information terminal including a housing7051, a display portion 7052, operation keys 7053, and the like. A modemmay be incorporated in the housing 7051 of the portable informationterminal illustrated in FIG. 21F. The semiconductor display deviceaccording to one embodiment of the present invention can be used for thedisplay portion 7052, so that a highly reliable portable informationterminal, a portable information terminal capable of displaying an imagewith high visibility, and a portable information terminal with low powerconsumption can be provided. The semiconductor device according to oneembodiment of the present invention can be used for an integratedcircuit for controlling driving of the portable information terminal, sothat a highly reliable portable information terminal, a portableinformation terminal with low power consumption, a portable informationterminal with high-speed driving, and a higher performance portableinformation terminal can be provided.

This embodiment can be implemented by being combined as appropriate withthe above-described embodiments.

This application is based on Japanese Patent Application serial No.2009-277078 filed with Japan Patent Office on Dec. 4, 2009, the entirecontents of which are hereby incorporated by reference.

1. A method for manufacturing a semiconductor device, comprising thesteps of: performing a first heat treatment on an oxide semiconductorfilm at a temperature higher than or equal to 500° C. and lower than orequal to 850° C.; adding oxygen into the oxide semiconductor film; andperforming a second heat treatment on the oxide semiconductor film at atemperature higher than or equal to 500° C. and lower than or equal to850° C.